Feature: information banner and installer of "install from Web"

This simply allows to install the joomla plugin that implements the Install from Web tab, and adds the useful events for that plugin.

Screendump ( http://awesomescreenshot.com/0c61sl0s36 ):
![3013228](https://f.cloud.github.com/assets/47402/1273588/85baa402-2d4d-11e3-8a9c-c551b53a1dcd.png)