global custom XML (SAX) decoder

In issue #53, we eliminate the type parameter of `Decoder<T>` in favor of a single decoder per feign instance.  This removes complexity around Set bindings in Dagger, moving the issue of type registry to decoder implementations.

This works in many libraries, such as gson, jackson, and jaxb.  They already have their own type-adapter registries, allowing a single Decoder instance to support an arbitrary count of types.

This pull requests implements this type registry concern for custom SAX decoders.

Existing use of RegEx decoders was limited to XML responses, so rather than doing something similar for regex patterns, we instead remove support for it and recommend SAX instead.

Once this is in, implementing #59 (which replaces internal usage of `Set<Decoder>` with a global Decoder) should be possible without eliminating feign's ability to custom parse XML.