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
Probably worth saying in a comment for the future test reader that 0xBA is equal to 62*3 and 0x18 is the remainder. Perhaps move the 3 out of check() and have it as a parameter to other buffer sizes can be checked?