FIX: Correct arithmetic operations

Fixis issues found in the discussion of #240:
Arithmetic operations are now done in their native datatype and only
casted if an overflow happens. This leads to better precision in results.
Also, hex strings are now allowed to have a sign (+ or -) if (and only if) they are
used in arithmetic operations.
Mh, you're right, your reading makes more sense. So I guess this is another case where Powershell's behavior deviates from a strict reading of the spec.

Powershell seems to do exactly what you suggest, so I guess that's the way to go here.