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This way it's possible to build using older MSVC (<13) that don't
support this yet. Macro is undefined in each file where it is used so it
should stack well with other libs and sources.

* Change node_map type from map<ptr,ptr> to vector<pair<ptr,ptr>>

Map nodes are now iterated over in document order.

* Change insert_map_pair to always append

Always append in insert_map_pair even if the key is already present.
This breaks the behavior of force_insert which now always inserts KVs
even if the key is already present. The first insert for duplicated keys
now takes precedence for lookups.

(Not stack<unique_ptr<T>> because it wasn't quite a stack; we needed to get the second-to-last element sometimes.)

This improves performance on the test.yaml attached to #158 by about
25% on my machine as compared to the previous commit (0.25s -> 0.20s),
as measured by `time build/util/parse < test.yaml > /dev/null`.

This improves performance on the test.yaml attached to #158 by about
35% on my machine (0.39s -> 0.25s), as measured by
`time build/util/parse < test.yaml > /dev/null`.

 - 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

Applied the patch given in jbeder/yaml-cpp#200 with the correct code
style.

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