Add hpack-test-case module.

Issue square/okhttp#931
First pass at adding hpack test implementation.

Issue square/okhttp#931
Refactor hpack tests for readability and extending.

Issue square/okhttp#931
Updated hpack tests to split good from bad.

Issue square/okhttp#931
Add roundtrip test case.
Just cleaned stuff up a bit in hpack tests.

Issue square/okhttp#931