Quality of life improvements and bug fixes

• You can now optionally specify an access token inside your config.json to make it easier to run tests.
• Object collections are friendlier to native python functions: print adaccount_object or print len(adaccount.get_ad_groups()) will behave as expected and not spit out random strings
• You can also now call .total() on object collections: print adaccount.get_ad_groups().total() will output total number of ad groups for that account (not just what's in the array)
• Pagination has been tweaked, pagination can be done with the following code:
  adgroups = account.get_ad_groups(fields=['name'])

while True:
    for adgroup in adgroups:
        print adgroup['name']
        if not adgroups.load_next_page():
            break

• Fixed method calls for endpoints that did not return cursor-friendly responses: get_conversion_stats() or get_stats() (which just return JSON objects).