Unsupported platforms no-op

@Sirupsen, @byroot, @jeromecornet 

This is a re-work of #7.

This adds a stub for the `Resource` class that effectively no-ops on non-Linux platforms (which is all we currently support). When calling `Resource#acquire` it simply yields.

The code currently prints a warning to STDERR when running on an unsupported platform. I think it's useful, but wouldn't be entirely opposed to removing it, if people think it's too noisy.