Add functions to run on plugin install/uninstall/upgrade

[martinkrcho]

It would be handy to have hooks in WP Activity Log that fire on plugin install/uninstall and during an upgrade procedure.

An extension could create a subclass of WSAL_Extension_Core and override couple of stub methods in order to do what it wants to do on the main plugin install/uninstall/upgrade.

We should discuss if this is enough. Does it make sense for the extension plugin to do anything without the main plugin?

[DannyWPWS]

As far as I see it, the extension should only do one thing if WSAL is not running, and thats to tell the user to activate/install it - though Robert may think of some cool thing we could do in addition :)