implement drone.io integration for both branches: master and trunk

[Sorin Ionuț Sbârnea]

It would be extremely useful, and easy, to enable builds on drone.io but this can be done only by a project admin.

I already tested it on one of my forks: https://drone.io/bitbucket.org/sorin/jira-dvcs-connector/1

Commands to be run:

sudo sh -c 'echo "deb http://sdkrepo.atlassian.com/debian/ stable contrib" >>/etc/apt/sources.list'
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys B07804338C015B73
sudo apt-get update > /dev/null
sudo apt-get install atlassian-plugin-sdk
atlas-mvn clean install -DskipITs=true