Add ThriftDocumentation annotation and create annotation processor to save javadocs

Adds a ThriftDocumentation annotation that can be applied to services, structs, and methods.  Also introduces a JavaDocProcessor annotation processor that will save javadocs on ThriftService- and ThriftStruct-annotated classes to specially-named mirror classes with ThriftDocumentation annotations, e.g., the javadocs for Foo are saved to Foo$swift_docs.