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
You should omit the parameter name _if it provides no documentary value_.  If it has no value - as in this example - why keep it when it's unnecessary?

What about: `consume(std::size_t byteCount);`