Changed HTTPAdapter to allow max retries to be specified when initializing.

Currently, to bump the number of ``max_retries``, you need to reach into the ``HTTPAdapter`` instance & manipulate ``HTTPAdapter.max_retries``. The other pool options are passed, with sane defaults, to the constructor so it makes sense this should be configurable in the same breath.

Passes all tests & updated documentation. Should be 100% backward-compatible, since it's a new parameter that's not being passed anywhere else & is being default to the same constant as before.