Improves emails content on branch creation & deletion
Changes version to 0.1
Use "private" styles for html email
Replaces mailer plugin

To be able to produce emails with just an html body the
former mailer plugin (Rhynofly/play-mailer) was changed
for a "new" one (typesafe/play-plugins-mailer) which
allows it, and the necessary code changes has been made to
adapt for the new plugin's API.
Sets application version for release
Changes application version for development