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.
Nope you've gotta add new interop tests by hand
On Jul 1, 2014 8:42 AM, "Jesse Wilson" <notifications@github.com> wrote:

> In okhttp-hpacktests/README.md:
>
> > @@ -0,0 +1,16 @@
> > +OkHttp HPACK tests
> > +==================
> > +
> > +These tests use the [hpack-test-case][1] project to validate OkHttp's HPACK
> > +implementation.
> > +
> > +TODO
> > +----
> > +
> > +*    Make hpack-test-case update itself from git, and notify if there are new
>
> done?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/square/okhttp/pull/976/files#r14411738>.
>