Add contributions attribute to User object

The repo contributors endpoint returns an extra ``contributions``
attribute, which this captures in the `User` objects returned from
``Repository#contributors``.

https://developer.github.com/v3/repos/#list-contributors
Guard against user.contributions being overwritten by refresh method
Expand comment for User.contributions attribute
Merge pull request #267 from jbarbuto/1.0-alpha

Add contributions attribute to User object