Offer the okhttp-urlconnection module in the 1.x branch.

This makes it possible to migrate to OkHttp 2 incrementally. Applications
can change their callers to use OkUrlFactory one-by-one, and then switch
to 2.0.
Merge pull request #859 from square/okhttp_16_jwilson_0522_trampoline

Offer the okhttp-urlconnection module in the 1.x branch.