Namespace the Joomla! Installer Application

This is something I've been wanting to do, to show as an example how to properly use namespaces within a Joomla! application. I figured the installer would be a good candidate, since it's never used by the end user except for the one time, but developers could still inspect the code closely, since it ships with every download of the CMS.

It's not perfect, but it's really close. I still need to make some updates to the JS calling some of the tasks. Namely, the ones surrounding the `InstallDababase_backup` command, because that task names creates a conflict with the PSR-0 autoloader, since it contains an _. This, however, is not really a big deal. I've used the namespaced installer to successfully complete a "normal" install locally on my Mac.

PR's against my branch to improve the code (if you find something that doesn't work in a certain scenario) and to help with the JS changes is of course welcome.

A big thanks goes out to @mbabker for this as well. Earlier this year (?) he converted the installer to the new MVC, which made the transition to the namespaces that much easier.