Revert builder-esque return types from OkHttpClient.

I'm still 100% fine with not merging this and just coordinating a Retrofit release to update it for OkHttp v2 support.
```
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] OkHttp (Parent) ................................... SUCCESS [  1.264 s]
[INFO] OkHttp ............................................ SUCCESS [  5.052 s]
[INFO] MockWebServer ..................................... SUCCESS [  7.109 s]
[INFO] OkHttp Apache HttpClient .......................... SUCCESS [  1.461 s]
[INFO] OkHttp URLConnection .............................. SUCCESS [  1.620 s]
[INFO] OkHttp Tests ...................................... SUCCESS [01:10 min]
[INFO] OkCurl ............................................ SUCCESS [  2.763 s]
[INFO] Samples (Parent) .................................. SUCCESS [  0.226 s]
[INFO] Sample: Guide ..................................... SUCCESS [  0.709 s]
[INFO] Sample: Crawler ................................... SUCCESS [  0.613 s]
[INFO] Sample: Simple Client ............................. SUCCESS [  0.689 s]
[INFO] Sample: Static Server ............................. SUCCESS [  1.360 s]
[INFO] Benchmarks ........................................ SUCCESS [  0.648 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 01:34 min
[INFO] Finished at: 2014-06-09T21:55:28-08:00
[INFO] Final Memory: 39M/201M
[INFO] ------------------------------------------------------------------------
```