Fetch all pages of pull requests

Pull requests can be spread across multiple pages, so share the
link navigation code that was added for reading comments.
Merge pull request #19 from bz2/github_prs_multiple_pages

Fetch all pages of pull requests

Pull requests can be spread across multiple pages, so share the
link navigation code that was added for reading comments.

@binary132 discovered that his proposals were being ignored, the github api documentation doesn't state prs get paginated, but the same scheme is used as is for comments.