QL Grammar & AST data types defined

more expressions tests added
more tests added
More tests added
refactored type system to include ..Type type objects and added typeOf methods to all expressions
Removed singletons from implementations
Some refactoring
Restored dependency checker and fixed all unit tests
Refactoring & bugfixes & more tests added
small fixes
small fixes
Test
more tests added
Merge branch 'bartolkaruza-fb' of https://github.com/software-engineering-amsterdam/sea-of-ql into bartolkaruza-fb
simplified type generation in symbol table and other minor refactoring
Added skeleton for the Android interpreter
Merge branch 'master' of https://github.com/software-engineering-amsterdam/sea-of-ql into bartolkaruza-fb