Make new request/response API public

[wsanchez]

Make new request/response API public. That would add to the package (__init__.py):

• From _imessage.py:
  • FountAlreadyAccessedError
  • IHTTPHeaders
  • IMutableHTTPHeaders
  • IHTTPMessage
  • IHTTPRequest
  • IHTTPResponse
• From _headers.py:
  • FrozenHTTPHeaders
  • MutableHTTPHeaders
• From _request.py:
  • FrozenHTTPRequest
• From _request_compat.py:
  • HTTPRequestWrappingIRequest
• From _response.py:
  • FrozenHTTPResponse

Before these become public, the following tickets need to be addressed:

• [x] #227: Decide whether to keep polymorphic text/bytes in headers API
• [x] #239: Use DecodedURL

[wsanchez]

This is mostly here to keep track of what needs to happen first.

[glyph]

We might want to chat a little bit about accessing the request objects via this new API vs. accessing them entirely through dependency injection. For example:

@route.get("/blah")
def blah(self, request):
    request.someNicerMethodForRetrievingAHeader(...)

vs.

@require(route.get("/blah"), header=Header("hello-world"))
def blah(self, header):
    print(header)

we are using an entirely request-less style at work and I'm really digging it.