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A flake8 configuration is included in setup.cfg and the checks are
executed automatically on Travis:

* Ensures a consistent PEP8 code style
* Does basic linting to prevent possible bugs

This adds a static local variable (in dead code unless actually needed)
in the overload code that is used for storage if the overload is for
some convert-by-value type (such as numeric values or std::string).

This has limitations (as written up in the advanced doc), but is better
than simply not being able to overload reference or pointer methods.

Problem
=======

The template trampoline pattern documented in PR #322 has a problem with
virtual method overloads in intermediate classes in the inheritance
chain between the trampoline class and the base class.

For example, consider the following inheritance structure, where `B` is
the actual class, `PyB<B>` is the trampoline class, and `PyA<B>` is an
intermediate class adding A's methods into the trampoline:

    PyB<B> -> PyA<B> -> B -> A

Suppose PyA<B> has a method `some_method()` with a PYBIND11_OVERLOAD in
it to overload the virtual `A::some_method()`.  If a Python class `C` is
defined that inherits from the pybind11-registered `B` and tries to
provide an overriding `some_method()`, the PYBIND11_OVERLOADs declared
in PyA<B> fails to find this overloaded method, and thus never invoke it
(or, if pure virtual and not overridden in PyB<B>, raises an exception).

This happens because the base (internal) `PYBIND11_OVERLOAD_INT` macro
simply calls `get_overload(this, name)`; `get_overload()` then uses the
inferred type of `this` to do a type lookup in `registered_types_cpp`.
This is where it fails: `this` will be a `PyA<B> *`, but `PyA<B>` is
neither the base type (`B`) nor the trampoline type (`PyB<B>`).  As a
result, the overload fails and we get a failed overload lookup.

The fix
=======

The fix is relatively simple: we can cast `this` passed to
`get_overload()` to a `const B *`, which lets get_overload look up the
correct class.  Since trampoline classes should be derived from `B`
classes anyway, this cast should be perfectly safe.

This does require adding the class name as an argument to the
PYBIND11_OVERLOAD_INT macro, but leaves the public macro signatures
unchanged.

- ICPC can't handle the NCVirt trampoline which returns a non-copyable
  type, which is likely due to a constexpr/SFINAE issue. This disables
  the test on that compiler so that at least the rest can be tested.

The C++ part of the test code is modified to achieve this. As a result,
this kind of test:

```python
with capture:
    kw_func1(5, y=10)
assert capture == "kw_func(x=5, y=10)"
```

can be replaced with a simple:

`assert kw_func1(5, y=10) == "x=5, y=10"`

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.