Adds support for installation as a must-use plugin

Pods breaks if installed as a must-use plugin and activated using a proxy loader ([codex notes on this] (http://codex.wordpress.org/Must_Use_Plugins)).

This fix detects how Pods is installed for instances when the regular plugins directory constant `WP_PLUGIN_DIR` is used.

The only instances left are in the Github Updater class (`includes/updater.php`), but those are not relevant since must-use plugins are not included in update checks.