Add getPullRequests method (not yet implemented)

                   
|Q            |A  |
|---          |---|
|Bug Fix?     |n  |
|New Feature? |y  |
|BC Breaks?   |n  |
|Deprecations?|n  |
|Tests Pass?  |n  |
|Fixed Tickets|   |
|License      |MIT|
|Doc PR       |   |
                   

Depending on merge of gushphp/gush#120