Removed unused imports.
Fixed bug in ANTRL-syntax: used '<=' instead of '-'.
Expanded the tests for the if-statement parser.
Added check if the condition of an if-statement is of type boolean.
Added a unit test for the 'CompositeFormElement' class.
Moved the ASTNodeVisitor to the 'ast' package.
Added and implemented a visitor for the 'Type' interface.
The typechecker method now returns a list of errors.
Fixed a bug: comparing types of variables failed. Replaced with .equals().
Added the sample from the course slides.
Merge branch 'JessevanAssen-master'
Merge branch 'master' of https://github.com/software-engineering-amsterdam/sea-of-ql into bartolkaruza-fb