Network interceptors.

These exist between the OkHttp features (gzip, response cache, retry, route
planning, etc.) and the network server. They can be used to rewrite the response
headers and body from what the server actually returned to what the client wants.

For example, the interceptor could inject a missing 'Cache-Control' header to make
caching work better.
Merge pull request #1220 from square/jwilson_1222_network_interceptors

Network interceptors.