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
Its a very simple rule, subsequent continuations are indented an additional tab stop from the previous line unless it is the last line in a `for`, `while`, or `if` with a one statement body in which case it is indented two tab stops.