Build step to set Github commit status as "pending"

add new build step which marks the relevant commit as “pending” using the github status api.

fixed webhook login to work with github enterprise, enabling calls to status api to succeed for both pending and final build statuses
<code>getLastBuiltRevision()</code>  and <code>getSha1()</code> are nullable. It is preferable to check them to avoid crashes like JENKINS-23641 ( #46  )