Allow trusted user to schedule pull requests for testing

Currently, the github:checout_pull_request job requires that
the trusted admin who can verify which pull requests should
be tested can only be the same as the user used for authentication.

This commits adds a seperate key to the github_auth file that can be
used to set a seperate admin
Merge pull request #49 from bodepd/truster_admin

Allow trusted user to schedule pull requests for testing