Collection Item Expressions

Added more support for translation of items with collections by:
- Renamed existing 'itemTranslation' to 'itemClass' on CollectionProperty
- Added new 'itemExpression' attribute on @CollectionProperty.
- Added initial support for translation to value types, starting with java.lang.String.
- Added support for item expressions on getEachTranslation()
- Used SourceProperty to apply the item expression, so it can be used with or without MVEL.
- Tests for collection translation using itemClass and itemClass + itemExpression.

Closes #90, #82.