Allow registration of custom readers, writers (to support 3rd party, custom types)

[cowtowncoder]

Currently there is no way to add custom handlers for reading or writing values; only basic JDK types (basic scalars, java.util.Collections, java.util.Maps, arrays, enums) and Beans are supported. Although we can add some more JDK types, it would be useful to allow custom readers / writers to support:

• Custom value types that do not conform to Beans (f.ex serialize as some other JSON value that JSON Object; or use different naming convention)
• 3rd party types (like Guava collections)

For 2.10 this could be done by adding new abstract -- ReaderWriterProvider -- configurable via JSON, that allows lookups for:

• ValueReader for deserialization
• a new abstraction for serialization