Add API to RKObjectMappingMatcher to be able to supply a dictionary of values to object mappings, so a value in a source map can do a single lookup to find the corresponding RKObjectMapping, instead of doing an iterative search through a number of individual RKObjectMappingMatchers.  To support the -objectMappings method of RKDynamicMapping, this required adding a new method to RKObjectMappingMatcher which can supply an array of possible RKObjectMappings to be returned.  Additionally, made that method return a constant array instead of creating a new one every call, as it gets called during the mapping process, which should further help performance when using dynamic mappings.  Also added an API so that a list of RKObjectMappings can be passed in with an evaluation block, again to better support the -objectMappings method.
Update code to add a block matcher rather than change RKDynamicMapping directly, which should be more flexible anyways.
Addressed a couple of pull review comments -- updated some docs, and changed a local variable name
Change init methods to use instancetype per PR request
Merge pull request #2093 from carllindberg/2065-DynamicMappingEnhancements

RKDynamicMapping enhancements