NH-2401

Implementation and simple unit test
https://nhibernate.jira.com/browse/NH-2401
@oskarb @hazzik: Debugging the code, it is clear that the parameter type is being correctly set, but I'm not sure how to do it in a unit test, have you got any suggestions?
Like this:

public static decimal MappedAs(this decimal parameter, NHibernate.Type.IType type)

?
Sure... it may make sense... want me to update the pull request?
BTW, not related, but why don't we also support, at least, Convert.ToDateTime() in a LINQ query?