Add Gabriella Chronis's SSH public key

Before this commit the `puppetlabs-sshkeys` repository
did not have an autorized public key for Gabriella Chronis.
This commit adds a public key for the user `gabriella` to
the directory `authorized_keys`.
Merge pull request #60 from gchronis/add_gabriellas_sshkey

Add Gabriella Chronis's SSH public key