Fix ResponseCacheTest for Android

Pull #1011 broke ResponseCacheTest on Android.

The test was asserting a header generated
using a copy of the old broken implementation.

The test now just uses HttpDate, which now has its own
tests.
Bump Okio dependency.
Merge pull request #1020 from square/jw/okio-update

Bump Okio dependency.
Merge pull request #1018 from nfuller/FixResponseCacheTestOnAndroid

Fix ResponseCacheTest for Android
Initial web socket API.
Implement message reader.

Currently the reader skips control frames.
Implement message writer.
Hook up the web socket reader and writer together.

Add a simple real-world recipe which talks to an echo server.