Add hpack-test-case based tests

For isssue square/okhttp#931 this creates a submodule referencing http2jp/hpack-test-case, and runs the decoder across each of the directories for interop testing.  Many of these fail, so they're in an @Ignored bad test.

Another test added does a round trip of headers through our encoder and decoder -- this test is not quite as effective as having known good output of our own in http2jp/hpack-test-case so that's next.
nit: weird to have toString anywhere other than last