Support api v2 using the same interface

HipChat::get_client now used as a factory to return an api version
specific implementation.

BasicHipChatClient - base class for keeping our php interface standard
across each version

HipChatV1 - The class responsible for translating the V1 spec into our
interface.

HipChatV2 - The class responsible for translating the V2 spec into our
interface.

Updated README.md to reflect the new HipChat::get_client factory method.

Updated example.php to pass token, version and api target and to use the
HipChat::get_client factory method