Improve @apimethod with types and docs.

This uses the working bits of WSME to handle data types for request and
response values.  And it introduces a new Sphinx extension to handle
documenting API endpoints.
Document REST API types
Add support for cross-referencing types and endpoints
refactor type display, and link all types to their definitions
verify that everything is documented
remove TODO item
document how to document api stuff
Merge djmitche/build-relengapi:issue29 (PR #29)
Merge in api changes from e1cedee
Support apidoc in slaveloan