Add common dialect functions as extension methods for QueryOver

Some of the common functions (that are registered in most, if not all Dialects) can be used in ICriteria using SQL-projections.

Extensions methods can be added for these to allow queries along the lines of:

s.QueryOver<Person>()
.Where(p => p.BirthDate.Year() == 1970)
.List()

... which can be translated to:

.Where(Restrictions.Eq(Projections.SqlFunction("year", NHibernateUtil.DateTime, Projections.Property<Person>(p => p.BirthDate)), 1970))

... and in turn to:

datepart(year, this_.BirthDate) = @p0;