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.

----
Update: Oh boy, one thin g lead to another to have proper functionality and tests, and here we are, with a lot of more features integrated:
- array concatentation with "+"
- array replication "*"
- hashtable concatenation with "+"
- accessing hashtables via index expression
- support for multidimensional arrays
- support for unary literal arrays ( as ",2")
- slicing via index, i.e. accessing multiple elements from a list/array/string/hashtable
- negative indices for arrays, lists and strings (-1 accesses the last element, etc)
- PSObject cannot wrap null, just be null itsel. according fixes
- a lot of tests
- maybe some more stuff I just forgot.

---
Update 2: Added type conversion support.