Pagination, EdgeIterator improvements

Summary: Improved pagination using the Python SDK. Made some improvements to working with the EdgeIterator object.

Test Plan:
Create an object that retrieves children and try the following:

    account = AdAccount("act_<AD_ACCOUNT_ID>")
    adgroups = account.get_ad_groups()

    print len(adgroups) # This works now
    print adgroups # This spits out pretty JSON instead of <EdgeIterator instance#4280t294 BLAHBLAHBLAHBHA>

I can paginate using the following code:

This is, unfortunately, Pythonic. Can't find a clean way to do do/while without a
`while True` in my code which I admit would keep me up at night.

    adgroups = account.get_ad_groups(fields=['name'])
    while True:
        for adgroup in adgroups:
          print adgroup['name']

        if not adgroups.load_next_page():
          break
Made it easier to bootstrap and run tests + interactive by allowing user to specify access token in config

Summary: Made it easier to bootstrap and run tests + interactive by allowing user to specify access token in config

Test Plan: Define access_token in config