Additionally, change preview media type to documented custom media type
for https://developer.github.com/v3/orgs/teams/#check-if-a-team-manages-a-repository

Fixes #262.

Change-Id: Ie9acf859346fce6b797b039d35b17e46f404e2a3

- Add new options struct to AddCollaborator (BREAKING CHANGE)
- Add Permissions field to User struct, which gets populated when
  calling ListCollaborators

This includes all of the methods mentioned in this blog post:
https://developer.github.com/changes/2014-03-18-paginating-method-changes/
as well as a handful of others I found.  Sadly, this is a breaking
change for many of these methods because it changes the method
signature, but with yesterday's API change these methods are now broken
anyway. :(

this change also includes some additional minor code cleanup

fixes #58