added option to cursor() to yeild "page" - ie: iterators over each page returned by twitter

When using cursors of the twitter api, results come back in "pages", with each page returned from a separate api call. If you want to track how many api calls you're making (not a bad idea if twitter's rate limit status is being unreliable, as in my case), it's handy to know how many calls a cursor is making as you iterate through it.

This pull request adds an option to `returnPages = False` the `cursor()` function. If set to `True`, `cursor()` yields an iterator for each page returned by twitter.