Upgrade to HttpClient 4.3.x.

Upgrade to HttpClient 4.3 and Guava 16.

Notes: 
* The only reason I added the _connectionManager_ and _defaultRequestConfig_ properties to `JestHttpClient` is to allow `JestClientFactoryTest` to keep doing what it has always been doing.
* Callling `JestClientFactoryTest.getObject()` without a config now uses HttpClient's default connection manager, which is the `PoolingHttpClientConnectionManager` instead of the `BasicHttpClientConnectionManager`.