[WIP] refactoring of the JDTTreeBuilder class

Some parts of spoon are really messy. That makes it harder as it has to be to understand how spoon builds the project model. A refactoring makes it easier to improve and expand the functionality. (for example to implement lambda expression from java 1.8 )

This is just a work in progress. I want to hear your opinion ( @monperrus ) about it first. Do you want to have a refactoring?
What don't you like at this first little work?