Support pagination for issue labels API

The Github v3 API supports pagination when retrieving issue labels, through the standard ``page``and ``per_page`` parameters [1]. This is true even though the parameters are not explicitly mentioned in the - rather brief - documentation of the issue labels API call [2]

[1] https://developer.github.com/guides/traversing-with-pagination/
[2] https://developer.github.com/v3/issues/labels/