Add GitHub issues handler

This is the initial commit of the GitHub issues handler. Its first functionality is the ability to list the open issues on a repository:

`gh issues PagerDuty/lita-github`

You can also pass options to it:

`gh issues PagerDuty/lita-github state:closed`
change method signature from to_i_if_number to to_i_if_numeric
Merge pull request #28 from PagerDuty/repo_issues

Add GitHub issues handler