Updated .gitignore for IntelliJ IDEA project files
Partial fix for #158 .

When an element is compared it now first checks if the other element has the 'type' property, avoiding a MissingPropertyException. When a choice is compared it now first check if the other entity has the 'elements' propery, avoiding a MissingPropertyException.

A Difference is created on both occasions, but without a proper message.
Merge pull request #163 from ithena/158-fix-simple-to-choice

Partial fix for issue #158 where the compare algorithm fails when an Element has changed to a Choice or vice versa