A Few Updates

Hiya,

Had to change a few things to use this plugin with a client and thought I would send them your way.

The big one: switch out all static methods for an `instance` and `init` scheme.   This is simply personal preference.

I also added some statements at the top fo the PHP files that prevent them from being loaded directly outside of WordPress.

The `process_csv` callback will only fire on the plugins admin page as it should instead of everywhere.

I also tested this with WP3.5.1 and it works fine, updated that in the readme.