Support new merge methods (squash/rebase) for Github Pull Requests

[NicholasBoll]

Note: This is part of a developer preview, so it might be too early to implement.

According the version 3 of the github API, new merge methods (squash and rebase) have been added, merge being the default: https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button

In GHPullRequest#L294, merge_method could be added to support different merge methods. Either the merge method could be overloaded to support different merge methods or methods for squashing and rebasing could be added.

[greggian]

Note, merge_method is now out of preview. PR updated accordingly.