Add @gapproperty

[fingolfin]

This would be identical to @gapattribute except it would add a type assertion to the return value of the getter function, so that Julia code calling the wrapper could be type stable.

In other words, this:

@gapproperty isperfect(G::GAPGroup) = GAP.Globals.IsPerfectGroup(G.X)

would be equivalent to this:

@gapattribute isperfect(G::GAPGroup) = GAP.Globals.IsPerfectGroup(G.X)::Bool