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
How can you define the member variable _before_ the class itself?!?

I don't like using `_` as a type name.  It's surprising to most C++ programmers - it makes it hard to glance over the class and see the uses of the templating class - but much more important, you have a chance with the name to say something about what sort of class you're expecting here.  And I really don't know what sort of class `_` can be.