Add custom key serializer to formats. It will be used to serialize/deserialize custom map keys into Strings to be compatible with the JSON specification.
Modify Extraction to serialize/deserialize map keys other than Strings.

* Default support for different types of integers (long, short, int, java versions of those), symbols, and dates
* Use custom key serializers when none of the above types apply
* Test coverage/examples in MapSerializationExamples
Merge pull request #129 from fedeoasi/map-custom-keys

Support maps with arbitrary keys (other than String)