NOS-84/ Allowing missing tests

Adds the ability to run the plugin without failing the build when tests are not present, instead the build will be declared unstable.

![screenshot from 2014-12-04 14 37 58](https://cloud.githubusercontent.com/assets/1848238/5299896/2c8315a2-7bc3-11e4-94f3-17b2d8118fba.png)
my bad Xavi I read unstable and thought this was the line and that was your point