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
That isn't clearing it up any.  :-)  Why not draw a picture of the string and indentify in that picture what the numbers 2, 5 and 7 mean - if you can't come up with documentary names that explain "why 7".

Here's the thing - if you came back and said, "Whoops, that should have been 8!" I'd have no way to tell which one was right.