Support untyped methods

Inspiration from https://github.com/mono/mono/commit/4d4e3c3d5b273149f66abe5605e15fce11713f55

This emulates the as3 behavior with 1 exception: if you specify a
return type of '*', but not all code paths return a value, PlayScript
will consider this a compiler error instead of automatically returing
undefined in those cases. I think this behavior is favorable.
Add /psstrict compiler flag for PlayScript "strict mode"

Strict mode defaults to on. When enabled, type definitions are required
for functions and class variables.
Update .gitignore to match upstream
Fix warnings building mcs.exe