updated fixie to support delegate fields, fixes issue #5

hey, i went ahead and took a little time tonight to familiarize myself with the way fixie works under the hood and figured i'd give adding this feature a shot.  all existing tests of fixie pass, though no new ones have been added.  I can add some around the new `CustomCaseFilter`, but wanted to see how you felt about this first.  the only change that is breaking is around the Cases property in the convention class.  you cannot do `Cases.Where(...)` anymore as Cases is now of type `ICaseFilter`.  this is a very narrow interface, and all of the chaining support had to be removed to accommodate it.  instead you now must new up either a `MethodFilter` or a `CustomCaseFilter`.