JED info link in extensions installer and 1-click installer of "install from Web" plugin

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)

Screendump of setting to switch off ( http://awesomescreenshot.com/06e1sw6t56 (old one: http://awesomescreenshot.com/0351sl1902 ):
![3027701](https://f.cloud.github.com/assets/47402/1284630/f6726bf2-2f97-11e3-8488-427736f372ea.png)

Related FR: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32175&start=0