Add documentation about how to configure Maven to use a http proxy

We have a subscriber who have trouble with the tutorial as his Eclipse cannot download log4j and other needed JARs
http://fusesource.com/issues/browse/DEV-3342

We should have some information in the Fuse IDE installation / tutorial guides that talks about how to setup a HTTP proxy for Maven.
I dont know if Eclipse have some settings for that.

Here is a guide for pure Maven to do that in the settings.xml file
http://maven.apache.org/guides/mini/guide-proxies.html