OkHttp HPACK tests

These tests use the hpack-test-case project to validate OkHttp's HPACK implementation. The HPACK test cases are in a separate git submodule, so to initialize them, you must run:

git submodule init
git submodule update

TODO

• Add maven goal to avoid manual call to git submodule init.
• Make hpack-test-case update itself from git, and run new tests.
• Add maven goal to generate stories and a pull request to hpack-test-case to have others validate our output.