* List reactions of issue
* Create issue reaction
* List reactions of an issue comment
* Create a reaction on an issue comment
* List reactions of a commit comment
* Create a reaction on a commit comment
* List reactions of a pull request review comment
* Create a reaction on a pull request review comment
* Delete reaction
* Renames IssueReaction to Reaction
* Fixes docstring in Reaction class
* Removes issue_url on Reaction class
* Fixes string concatenation that does not work on python 2.6
