Convert commands to classes.

This is the beginning of a proof of concept for converting Drush commands to classes per https://github.com/drush-ops/drush/pull/532#issuecomment-38189227. I'm beginning by creating a `CommandInterface`, imagining that individual commands would implement it. I started by just creating a method for each of the major elements returned by `COMMANDFILE_drush_command()` and one for each of the command hooks. Does this seem like the right approach? Should I continue?