Drop OkHttp 2 APIs for 1.5 release.

This drops async stuff: Request/Response/Dispatcher from public APIs.
Also hides support classes: CacheControl, Failure, Headers.

Also drops okcurl (which needs Request + Response).

New types in 1.5: Protocol, Handshake.
Merge pull request #576 from square/okhttp_15_jwilson_hide_more_2x_apis

Drop OkHttp 2 APIs for 1.5 release.