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
Make Canonizer more flexible

This exposes a few things to configuration via kwargs:

 - how to extract the scheme from the request
 - how to extract the hostname from the request
 - whether the redirect should be permanent
Merge pull request #382 from gratipay/flex-canonize

make Canonizer more flexible
Start ripping out configure-aspen.py; #373
Rip configure-aspen.py out of doc