[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
I removed it because I got errors with that generic method type.  Every time the convert value was invoked was the following message: 

"incompatible types; inferred type argument(s) java.lang.Object do not conform to bounds of type variable(s) E
[ERROR] found   : <E>java.lang.Object
[ERROR] required: java.lang.Object"

I solved it within the convertValue method by invoking the method this way: 
```this.<E>convertValue(field.getDefaultExpression());```
But I didn't find a way to solve it within the method getElementValue(String). Do you have any suggestions for solving that? 

By the way it seems to work for you because you compile the project with the jdk 7, right? The error just appears while compiling it with the jdk 6