Added 1-click install button for addons when activation codes are found

If activation codes are found in the options table or via the acf_settings filter then a convenient one-click **Install** button is placed on the Add-Ons page instead of the **Purchase & Install** button.

![Add-Ons screenshot](https://f.cloud.github.com/assets/9314/389735/9c012ab4-a71b-11e2-8373-771499075f2d.png)