Adding MIT License file

Adding explicit license file, originally taken from https://github.com/jenkinsci/jenkins/blob/master/LICENSE.txt with the only change being the removal of the Copyright line.

This is motivated by our corporate open source usage policy and is based on information found at the link below that if no explicit license is specified in either file headers, the license section of the POM, or a license file at the root, then "your code will be assumed to fall under the MIT license terms."

https://wiki.jenkins-ci.org/display/JENKINS/Before+starting+a+new+plugin

If you'd like anything particular in the copyright line or any changes to be made, please just let me know and I'd be happy to update it.