Override Login.php Template from your active theme

This adds the field Override Template option to admin and updates maybe_show_login to load the template specified from your active theme. Checks for file to make sure it exists first, if not will default back to plugin template. Path is relative to your theme's root.

![screenshot 2013-10-16 18 22 41](https://f.cloud.github.com/assets/12783/1348548/bc1bab8c-36ca-11e3-9b39-ac2bce6e2b1a.png)