remove use of Set bindings for type adapters

In order to support issue #53, we need to eliminate cases where Set bindings were used for type adapters.  Libraries and frameworks such as gson, jackson, and jaxb have their own type-adapters.

Feign also supports custom decoding.  There are 2 cases of this: SAX and RegEx pattern decoders.  This pull request ensures each of these implementations have means to register types.

With this in, implementing #59 (which replaces internal usage of `Set<Decoder>` with a global Decoder) should be possible without eliminating feign's ability to SAX or RegEx decoding.