Implement Mustache templates for messages

A number of people in issue #10 have asked for the ability to customise the messages, such as adding build parameters etc.  By using Mustache templates for build messages this can now be done.

This is quite a large change, and probably not as heavily tested as it should be. It probably needs some unit tests too.  I am also not a Java developer, but "know" the language, so I might not have done everything optimally.

The README has been updates with information as to how the templates work, and the defaults should keep the messages the same as they were pre-Mustache.

This doesn't actually solve the original issue as it doesn't add the git branch name, and if the branch name isn't available through the accessible instance variables then it won't, I haven't checked.

It's probably a good start though!