Breakout Requests Functionality to New Adapter Module

[jeffwecan]

This PR is the first set of changes (technically second) meant to move #95 forward. It aims to move almost all calls into the requests module to a new adapter class.

• This allows some code to be moved out of the Client class. The ultimate goal is to greatly reduce the amount of code that needs to be maintained directly in the Client class.
• It also enables end-users to more easily override or modify the behavior surrounding requests without needing to subclass the Client class directly.
• Existing call sites have been left in place. However the now will include deprecation warnings for any users that had been calling these (mostly private) methods directly.