Add STASH_REPO_NAME and STASH_IS_ADMIN to the hook environment

I was looking to use these in my external hooks: 
   • STASH_REPO_NAME - useful for logging and for rejected push responses.
   • STASH_IS_ADMIN - useful for allowing certain users to push by permission (in some cases)

I did **not** bump the revision in the pom.xml file.