Fix convertLinks

I've added support for running unit-tests using `gradle test`, a test for `OrgNode2Html.convertLinks`, and finally fixed `convertLinks`; it wasn't correctly recognizing HTTP urls.

There are a couple of caveats, I'd be happy to address them if you think they're problematic, and you can point me in the right direction:

 - I've never used Gradle before, so there's a chance this could be nicer. I followed https://github.com/JCAndKSolutions/android-unit-test in setting up unit-testing.
 - There's no support for marking a directory as a test source, it needs to be done manually.
 - I don't really understand what the original code I changed in `convertLinks` was supposed to do; I'm assuming the extra stuff in the regex was not intentional. If it was, now at least it's easy to test it.