Return NaN instead of 0 for addition with undefined

The other arithmetic operators were doing the right thing, but addition
was missed
Make sure appending null to a string appends "null"

In C# casting null to a string results in an empty string, but in
ActionScript it gets the string value of "null"
Fix arithmetic operations involving null/undefined
Add support for unit tests in .cs files

Useful for debugging failures in the mcs test suite