feat(handler) b3 single header

[kikito]

Implements #21

[codefromthecrypt]

I'd be happy to review, but it is easier on the eyes for me if some assertions are made with actual headers and anticipated results. ex pre-canned headers

https://github.com/openzipkin/brave/blob/master/brave/src/test/java/brave/propagation/B3SingleFormatTest.java

this could be in addition to the integrated tests and just a suggestion.

[kikito]

@adriancole thanks for the suggestion. I have added some unit tests for the header parsing, which test all the new b3 single header cases more exhaustively, in isolation from the rest of the app.

[codefromthecrypt]

looks good thanks!