Update facebook.py

[allfro]

Added iterating methods for paging

[martey]

I think my comments on pull request #37 still apply - these new next and previous methods are of limited utility, and there is unnecessary duplication from the existing request method.

Since Facebook looks to be deprecating previous-next pagination, I think this is not the right time to implement something like this.

[rbpasker]

martey -- you've closed two pull requests with pagination solutions. could you please add something to docs that explains exactly how you expect people using the API to do paging?

( #37 )

[vmassuchetto]

+1 for documenting the paging functionality. It's confusing how to do this with this API.

[skyler]

I'm sorry if I've missed this elsewhere, but how should we do pagination using this API? Should I make an HTTP request using the URL returned in next and previous?