Feature/hooks

This pull requests implements a series of new features:

1) The ability to disable multipart parsing from a configurable option

2) The ability to set a maximum size for form-urlencoded requests

3) A hook that can be override by the web application developer for generating the content buffer (see https://github.com/hellais/GLBackend/blob/62285a17fcbd6743f42173b2e7e922255dc3fc5b/globaleaks/backend.py#L24 for an example use case)

4) Support for handling `Content-Disposition` and `Range` HTTP Headers

If you want to discuss more about this merge I am available on IRC on #twisted (Freenode) or on #globaleaks (OFTC).