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?
Hooray, you've learnt how to use line comments... sorry, just trying to joke.

>Like this:
>public static decimal MappedAs(this decimal parameter, NHibernate.Type.IType type)

Probably 

`public static decimal MappedAs(this decimal parameter, NHibernate.Type.DecimalType type)`

But I see now, that this is a bad idea.

>but why don't we also support, at least, Convert.ToDateTime() in a LINQ query?

Just because (c) my 4yo. Just because we forgot to support it.