Add support for a header based protocol

[thecodeboss]

There are a few thrift protocols like TTwitter from finagle, or THeader from fbthrift that allow for things like tracing. We should consider adding support for one or all of these at some point.

This is very related to #343, as being able to add middleware to a thrift stack would make this much more straightforward and interchangeable. In fact, #343 is probably a prerequisite.