Update to New Instance/Init Scheme

`instance` method creates a new instance of the class
`init` hooks that instance's `_setup` method into plugins loaded.

All other actions get added from that `_setup` method.

See: http://wptip.me/classes-in-wordpress-plugins-themes

Other stuff:
- Add `!defined('ABSPATH') && exit;` statements to the top of the PHP
  files. Keeps them from being accessed outside fo WordPress
- Remove the `process_csv` hook from `init`, and only fire it when the
  import page loads
- Move the plugin text domain into a `plugins_loaded` hook
Remove Additional Capability Check

WordPress takes care of checking capabilities before a page get's
loaded. No need to do it yourself.

Also add a `screen_icon` for prettiness purposes.
Works perfectly on WP 3.5.1