Add basic junit format support

Hey Nate!

I'm sending you a rough prototype to support generating valid junit output when using `check_sf --junit result.xml`

I've been working on this with @cgoncas over the week, and the idea is to enable `check_sf` in your pulls (or any frontend reviewer). Then, we should be able to present the results on github the same way they currently do for Brian on https://github.com/brianchandotcom/liferay-portal-ee/pull/7152#issuecomment-58438701. They are currently working on some issues with the Github plugin, but we already configured Cristina's task so you could check the results in here: http://ci.liferay.org.es/jenkins/job/cgoncas/266/testReport/

About the PR itself, feel free to work around it any way you see fit. In addition, I think we should work on exposing a consistent test results data that any report generator could use. For example, we could consider grouping all code that potentially calls `trackErr`. The way I see it, all those validations could be considered as rules. It also would help us keep track of how many rules are run against a given file. 

What do you think?

/cc @ipeychev, @mdelapenya