Fix several problems with the parser

This request fixes the following:

The generated parser required manual editing, which made it tricky to update. The package-info.java file in src/gnu/prolog/io/parser noted that it would be better if this were not the case. The patches below mean that javacc generates the complete parser automatically.
The generated parser would only accept variable names of 1 or 2 characters in length (!? Maybe this is a difference in implementation between javacc5 and javacc6, but to me the grammar just looks completely wrong)
The generated parser did not correctly parse {} as an atom. (See #6 - this is the real reason I started digging into the code. The other 2 were necessary to get the parser to a point where I could actually fix this problem and test it)

drt24 / gnuprologjava