Ported Apache Client 4 to Jersey 2.0 connector implementation (including all the tests).

http://java.net/jira/browse/JERSEY-1705
Ported from Jersey 1.x. Tested with Apache HttpClient 4.2.3. All tests run fine.
I had to use a pooling client for two tests (testPostErrorWithEntity(), testPostVoid()) in HttpMethodTest to make them work. I hope I am not overlooking anything on these two tests.