add Lita::Handlers::GithubPR handler

This is a handler for managing pull requests on repositories. This initial commit only includes the ability to merge the PR.

* `gh pr merge #42 PagerDuty/lita-github`
* `gh shipit #42 PagerDuty/lita-github`
* `shipit #42 PagerDuty/lita-github`
improve PR merge error handling

There was the possibility of an uncaught exception if an invalid PR was provided. This catches that exception and replies indicating the error.

While doing this the cyclomatic complexity of `pr_merge()` got a bit nuts, so some business logic was removed from the method and put in `merge_pr()`.
make the method_disabled message more clear
Merge pull request #5 from PagerDuty/initial_github_pr_handler

add Lita::Handlers::GithubPR handler