More robust - fix 3 issues

The latest code fixes 3 issues:

https://github.com/boxuk/jslint-jenkins-plugin/issues/3 - JSLint and globals
https://github.com/boxuk/jslint-jenkins-plugin/issues/2 - generated XML can be invalid
https://github.com/boxuk/jslint-jenkins-plugin/issues/1 - NullPointerException running build

If you could merge this PR into the Jenkins fork , that would be great :-) Thanks

- Gavin Davies
Principal software developer
http://www.boxuk.com/