Stream support #57

At this time, cppcodec does not offer a good way of encoding or decoding a byte stream of unknown length. This is a significant shortcoming in the API and something usable should be added. The obvious choice here is support for the stream operators (<<, >>) that pipe from and to a cppcodec class. We'll likely want a new class because unlike the existing encode()/decode() functions, this one needs to track state as part of its member variables, i.e. the data source and possibly information about the current offset.

I'd like to make this a focus area for v0.3.