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)
https://github.com/novoda/junit-plugin/commit/e61fd41b699232f95778dd7259de6b25e13a5385

this diff is easier to see the change made