Added Activities endpoint and some QOL stuff for EdgeIterator objects

Summary:
AdAccount.get_activities() was previously broken
Updates to EdgeIterator are inline with some changes I made last week making
the EdgeIterator object friendlier to use, allows user to do:

campaigns = account.get_ad_campaigns()
print campaigns[0]

Test Plan:
campaigns = account.get_ad_campaigns()
print campaigns[0]