Change the wrapper varable name to avoid confusion.

 The old example was overloading 'congress', and it was a skitch
 confusing. Closes #17

 >>> from sunlight import congress
 >>> from sunlight.pagination import PagingService
 >>> paging_service = PagingService(congress)
 >>> print(len(list(paging_service.legislators(limit=55))))   # page more than a single page
 55

 Thanks for the report, Tom.