Make Travis CI build on Ruby 2.0.0 also
Replace some skipped tests with actual tests or explanations
Handle array :in arguments of enum type
Handle generic pointer-like :in arguments
Introduce ZeroTerminated to represent zero-terminated arrays

For now, it only works with :int32.
Make InPointer#from_array always zero-terminate its result

This makes ZeroTerminated work correctly on Ruby 2.0.0.