Add a toy HTTP backend.

[nud]

The basic concept of this backend is to forward TFTP queries "as is" to an
HTTP server, as a simple way to allow dynamic files generated on the fly for
each TFTP request.

RRQ are forwarded as GET requests.
WRQ are forwarded as PUT requests.

[shylent]

To be honest, I am very strongly against using synchronous network operations (HTTP requests) in the context of an asynchronous library, especially considering the fact, that there is a solid HTTP implementation in twisted itself.

I would replace the use of "requests" (and the uncalled for introduction of external dependency) with the use of twisted.http.client.

Perhaps, I'll find some time in the following week or two to do it myself.