Support for additive and multiplicative operators

These commits introduce the support for the "*", "-", "/", and "/" operators.
Beside the arithmetic meaning, also the string replication feature of the "*" operator was introduced.

Notes:
- The LanguagePrimitves was extended by some functionality defined in the Powershell spec.
- All operators use the same method for arithmetic conversion and execution of the operation.
- Tests from the specification were implemented as ReferenceTest cases.