Implemented New Method to Retrieve all Collaborators from a Repository

Hi. 

I have a very ordinary use-case: I need to check if a Github user has access to a repository. Unfortunately, the Github's API doesn't provide a way to do it directly. To do it, I need to get all repos and check in which repositories the user is in repository's collaborators list. I tried to implement it, but a found a little problem: the method getCollaborators just return the first 30th collaborators. So, I've implemented a new method to retrieve all collaborators from a repository doing pagination just like several other methods from this library. 

There is also, in this pull-request, fixes to some formatting issues and a unitary test-case to the new method.

Any question/suggestion, please, feel free to contact me.

Thanks in advance