Native extension aliases

## Fix bug #150 

We recently discovered that Rubinius installs the gem version with pre-compiled extensions. The internal guard logic assumed that the C-native build would only be installed under MRI. As a result we were experiencing unintended behavior on Rubinius. This PR addresses this issue by adding more robust guard clauses, attempting to ensure that C extensions are only loaded on MRI, regardless of what platform a particular interpreter reports.

Because the release of 0.7.0 was put on hold, the guard clauses are very brute-force. They can probably be cleaned up in a later release.

NOTE: It has been suggested that we consider using a gem called Redcarpet for the guard functionality. This gem was unknown to us until yesterday. At this time we have explicitly decided to not include an external gem dependencies. We will consider using Redcarpet in a future release.