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
The decision was made to make this a header-only implementation. My intention is to take beast in the direction of being header-only in a fashion similar to boost. Some things will still require linking (like the joyent HTTP parser, a few others). In some cases header-only implementations are unavoidable, such as when working with user defined types passed as template arguments, a common idiom in asio code). In this case the algorithmic heavy lifting is done in a function that must be templated. Therefore the class is made header-only. A benefit of header-only implementation is zero impact on the build commands, and no changes required to unity .cpp files. This increases the likelihood of adoption of beast.