Updated requests, pool improvements

* Updated docs to reflect latest requests code
* Alphabetized methods
* Changed pool to only allow close after `.finish()` has been called

cc @muffinresearch
Allow post-close requests in pools

This allows the pool to accept new requests after the pool
has been closed. These requests, however, don't count towards
the pool's promise and the pool simply acts as a pass-through
to the requests module. In addition, the pool's abort() method
will terminate these requests.

This fixes issues with pagination hard-failing.
Merge pull request #112 from mozilla/requests

Updated requests, pool improvements