Now it works with real xml files.

- It now works on real xml files.
- I separated the class Main into two classes: the Main class which is
used only for executing (aka showing the behavior of the program while
I'm coding) and the XMLParser which does, well, XML parsing.
- Refactored class Attribute.java into Variable.java, though no other
changes were made besides changing its name.
- I started adding JUnit test classes. For new there is just one,
though, the class ConstantsTest.java.