Add http::chunked_encoder:

This transforms a ConstBufferSequence into a new ConstBufferSequence whose
data is encoded according to the Content transfer encoding rules of RFC2616.
The implementation does not copy any memory but also does not coalesce
buffers into larger ones if the input data is in small pieces.
Reviewers: @rec @donovanhide
The trouble with having all the characters the same in that last test is that you can't really tell if you glitch and avoid filling in some areas.  

Could you perhaps use a repeating pattern like `abcdefghijklmnopqrstuvw`?  

Sorry for being paranoid, but I have experience with buffer copying algorithms that _almost_ work - in audio, the result is a little degradation, but in our case the result is a malformed response...