Adding Replacement serialization support

Adding a new data structure for storing the Replacements generated for a single
translation unit. Structure contains a vector of Replacements as well a field
indicating the main source file of the translation unit. An optional 'Context'
field allows for tools to provide any information they want about the context
the Replacements were generated in. This context is printed, for example, when
detecting conflicts during Replacement deduplication.

YAML serialization for this data structure is implemented in this patch. Tests
are included.

Differential Revision: http://llvm-reviews.chandlerc.com/D1422