Improvements to the Linux Install Section

I've improved the detail in the Linux install section that should make it easier for people like myself to install Sublime Text on a fresh install. This is mainly targeted at Ubuntu, but the first bit can be used on any platform.

The basis for this came from [TechNoReply](http://www.technoreply.com/how-to-install-sublime-text-2-on-ubuntu-12-04-unity/) So hat tip to them.