Theme & Plugin actions

It would be nice to be able to install, activate, deactivate and delete themes & plugins using WP Remote.

* [x] wp remote plugin-install
* [x] wp remote plugin-activate
* [x] wp remote plugin-deactivate
* [x] wp remote plugin-uninstall
* [x] wp remote theme-install
* [x] wp remote theme-activate
* [x] wp remote theme-delete