GitLab

 - Adds 'std=c++11' compiler flags
 - Replaces boost::type_traits with std::type_traits
 - Replaces boost::shared_ptr with std::shared_ptr
 - Replaces std::auto_ptr with std::unique_ptr
 - Replaces raw pointers with std::unique_ptr in ptr_vector, ptr_stack, and SettingChanges
 - Replaces boost::noncopyable with deleted copy and assignment operators
 - Replaces boost::next with std::next
 - Replaces boost::enable_if with std::enable_if
 - Replaces boost::is_convertible with std::is_convertible
 - Replaces ptrdiff_t with std::ptrdiff_t
 - Replaces boost::iterator_facade and boost::iterator_adaptor with std::iterator, borrowing the 'proxy reference' technique from boost
 - Removes Boost dependency from CMakeLists
 - Formats changed files using clang-format

Node::force_insert() uses convert<> to convert its key to a node.
Add a specialization for convert<Node>.

"const Node Node::operator[](const Key& key) const" changed from
returning new empty node if the key was missing in 0.5.1 to returning
a shared 'zombie' node in 0.5.2 to resolve a memory leak.

(Specifically 1025f76d was where this
was introduced)

This caused some regressions where this 'zombie' object threw exceptions
in some functions where the 'empty' object would not.

This change fixes the Node::as(fallback) method (to return the
'fallback' instead of throwing an exception) and the
Node::begin()/Node::end() methods to return default-constructed
iterators (so begin() == end() in such cases) instead of another
exception.

- Update the call to equals() in node_data::remove() to match the new implementation
- Add unit test for node::remove() to catch this type of bug in the future

Add EmitterStyle, which will allow sequence or map style (i.e., flow or block) to be preserved between parsing and emitting

Fix clang compiler settings, and properly set up warnings so they don't interfere with gtest and gmock

warning C4800: forcing value to bool 'true' or 'false' (performance warning)

for the node test, since it really doesn't make any sense in this context. (It's exactly what we intended with the "unspecified bool type".)