Added logic to the LabelFor<> Helper

Logic now includes a "*" in the label if the field has a required
attribute or is not able to be assigned null.
Refactoring required token logic

Move display logic into ExpressionParser.DisplayFor to reduce hits on
expression parsing.
Added a Global Setting for the token and whether or not to use it.
Merge pull request #4 from zackpudil/master

Required Tokens when display required properties