Adding first_page? and last_page? to CollectionMethods

@mislav for review

While implementing paging for an API I needed to set some headers based on the current page. It read so much better when I could send `first_page?` and `last_page?` to the returned collection from `paginate` to determine which links to return. 

Initially, I just monkey patched it for my project. Then I thought, maybe someone else would find this useful, so I've added the two additions in this PR (along with tests).