Allow installing MRI from a binary mirror

This downloads and installs a precompiled version of MRI from an RVM-compatible mirror if `--binary` is passed

The system auto-detection is a huge mess, and probably violating the style guide (several lines longer than 80 characters). It's mostly lifted from RVM's own system detection. I don't know if you want this somewhere else (if so, where?) or if you want me to shorten the number of supported systems, just let me know and I can modify.

As for the long lines, should I just "wrap" them?

This currently just fails if `--binary` is passed and no binary build was found, since that was the easiest to implement.