Fix #377: Reify body_parsers and make request.body dynamic

request.body is now the result of running the appropriate body_parser
(based on content-type header) over request.raw_body

Inlined the functionality of context.py into the dynamic_resource object
Make request body parsing lazy

Make body parsing raise UnknownBodyType (HTTP 415) if the body
is an unknown type
Merge pull request #379 from gratipay/issue377

Fix #377: Reify body_parsers and make request.body dynamic