Make a home for MockWebServer in OkHttp.

To build HTTP/2.0 support into OkHttp I'm going to simultaneously
build HTTP/2.0 support into MockWebServer. To make that possible,
I'm first going to fork MockWebServer and then combine it with
MockSpdyServer.
Merge pull request #261 from square/jwilson/home_for_mockwebserver

Make a home for MockWebServer in OkHttp.