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Disable most implicit conversion constructors

We have various classes that have non-explicit constructors that accept
a single argument, which is implicitly making them implicitly
convertible from the argument.  In a few cases, this is desirable (e.g.
implicit conversion of std::string to py::str, or conversion of double
to py::float_); in many others, however, it is unintended (e.g. implicit
conversion of size_t to some pre-declared py::array_t<T> type).

This disables most of the unwanted implicit conversions by marking them
`explicit`, and comments the ones that are deliberately left implicit.

Accept any sequence type as std::vector (or std::list)

Accept any sequence type as std::vector

Fix dynamic attribute inheritance in C++

`PyType_Ready` would usually perform the inheritance for us, but it
can't adjust `tp_basicsize` appropriately.

Add dynamic attribute support

This convenience function ensures that a py::object is either a
py::array, or the implementation will try to convert it into one. Layout
requirements (such as c_style or f_style) can be also be provided.

Permit creation of NumPy arrays with a "base" object that owns the data

This patch adds an extra base handle parameter to most ``py::array`` and
``py::array_t<>`` constructors. If specified along with a pointer to
data, the base object will be registered within NumPy, which increases
the base's reference count. This feature is useful to create shallow
copies of C++ or Python arrays while ensuring that the owners of the
underlying can't be garbage collected while referenced by NumPy.

The commit also adds a simple test function involving a ``wrap()``
function that creates shallow copies of various N-D arrays.

- This actually works with no changes, I just wasn't 100% convinced and
  decided to write a test to see if it's true.

extra python version sanity check at import time

Python 3.5 can often import pybind11 modules compiled compiled for
Python 3.4 (i.e. all symbols can be resolved), but this leads to crashes
later on due to changes in various Python-internal data structures. This
commit adds an extra sanity check to prevent a successful import when
the Python versions don't match.

This fixes an issue that can arise when forwarding (*args, **kwargs)
captured from a pybind11-bound function call to another Python function.
When the initial function call includes no keyword arguments, the
py::kwargs field is set to nullptr and causes a crash later on.

Re-add accessor bool operator

PR #425 removed the bool operator from attribute accessors.  This is
likely in use by existing code as it was the only way before #425 added
the `hasattr` function to check for the existence of an attribute, via:

    if (obj.attr("foo")) { ... }

This commit adds it back in for attr and item accessors, but with a
deprecation warning to use `hasattr(obj, ...)` or `obj.contains(...)`
instead.

Simplify base class detection for Eigen types

Add in casts for c++11s chrono classes to pythons datetime

Make the accessor interface more complete

`auto var = l[0]` has a strange quirk: `var` is actually an accessor and
not an object, so any later assignment of `var = ...` would modify l[0]
instead of `var`. This is surprising compared to the non-auto assignment
`py::object var = l[0]; var = ...`.

By overloading `operator=` on lvalue/rvalue, the expected behavior is
restored even for `auto` variables.