Adding Proxy support to Joomla

Same as #1685, but cleaned up and against staging.

### Issue
Joomla can't connect to update servers or any other extern URL if its webserver is behind a proxy. This is often the case in corporate enviroments.

### Solution
This PR will add new parameters to the global configuration which allows to specify a proxy server, port, username and password.
If enabled, it will restrict JHttp to only use curl by default and curl will be told to use the proxy settings.

#### Tracker
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30941