Re-use HttpClient in BoxBasicRestClient

Currently, we are creating a new HttpClient for every remote call
the SDK makes. This is very inefficient in that it leads to certain
actions like SSL negotiation to happen on every connection, instead
of once per route. This patch re-uses the same HttpClient for every
call, and uses a ThreadSafeClientConnManager to manage the pool of
connections.
Make member private
Merge branch 'master' of github.com:box/box-java-sdk-v2 into keepalive
Merge pull request #54 from box/keepalive

Re-use HttpClient in BoxBasicRestClient