[WIP] make code compiles with jdk 6

This is a work in progress for the issue #58 to compile the project with jdk 6 instead of 7. 
The code does not compile yet!

We've to discuss how the AutoCloseable interface shall be handled. The CtTry interface uses the AutoCloseable interface to set the resources which was specified with the try statement. But this interface doesn't exist in java 6. 

I think we just should use CtLocalVariable<?> instead of CtLocalVariable<? extends AutoCloseable>. What do you think? Hm