Squash notices that appear when WP_DEBUG is enabled

Running plugin version 0.2.3 on WordPress 3.6 with `WP_DEBUG` enabled I was getting two notices on my site:

![screen shot 2013-08-16 at 5 33 57 pm](https://f.cloud.github.com/assets/233836/978422/c46a90ee-06bb-11e3-92df-0fa88bd05baf.png)

This pull request updates the call to `load_plugin_textdomain()` to a newer (e.g. non-deprecated) approach and checks that `$_POST['cmd']` exists before attempting to use it for comparisons.