Pipp is now more boring

Comparing different parsing and tree transformation techniques was for me a major reason for starting work on Pipp. Therefore Pipp had until now support for different frontends:

* Antlr 3: Parsing and tree transformation with Java-based Antlr 3 * PHC: Take XML-output from phc and transform it with XSLT * PCT: Parrot Compiler Toolkit

Lately I found that I only worked on the PCT variant. So in order to keep it simple, especially for new contributors, I removed support for Antlr3 and PHC.

So, Pipp is now more boring, but other exciting things are coming.