Cherry-pick ResponseCacheAdapter.

commit 2a4b1c18a375a53bb19b966a464e5dc7de38399f
Author: Neil Fuller <nfuller@google.com>
Date:   Mon Feb 24 17:41:09 2014 +0000

    Adding ResponseCacheAdapter.

    This allows OkHttp to continue supporting java.net.ResponseCache,
    which is desirable for Android's HttpURLConnection
    implementation.
Merge pull request #600 from square/okhttp_15_jwilson_cherrypick_ResponseCacheAdapter

Cherry-pick ResponseCacheAdapter.