IdentityT to transformers

[Henning Thielemann]

Ross, thanks a lot for adding MaybeT!

Unfortunately I have just another wish. In a parser I like to show by 
types whether the parser can fail or not. To this end I have a type 
parameter for a monad transformer that can be instantiated either by 
MaybeT or by IdentityT. The only problem is, that there is no IdentityT in 
transformers package, yet.