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
Empty line please after 36.  Also, the topic sentence isn't clear:

1. Lose the "simple" - it doesn't help describe it.
2. "transformation" could be pretty well any change.

A first start:  Convert a `boost::io::const_buffer` to chunked transfer encoding.