[swift2thrift] Pass javadoc comments through

Adds a ThriftDocumentation annotation that can be applied to services,
structs, methods, fields and enums.
Also introduces a JavaDocProcessor annotation processor that will save
javadocs on ThriftService-, ThriftStruct- and ThriftEnum-annotated classes
to specially-named mirror classes with ThriftDocumentation annotations,
e.g., the javadocs for Foo are saved to Foo$swift_docs.

Original code by tim <twilliamson@gmail.com>, modified, bugfixed and
extended by me.