add per_page query parameter

This addresses #83. This doesn't include an option for requesting an unlimited number of pages.. I'm not sure if that's a good option (e.g., GitHub doesn't offer that in their API, do they?) and you mentioned needing to do some performance testing on that.

I added a single test for pagination on projects only, I wasn't sure what the right balance is between code coverage and robustness, etc. Let me know if you'd like me to switch up the tests and expand it past just the single example.