Added a file for mode-mappings.

This file will contain all of the code to append to the list

    auto-mode-alist

Which controls the mappings beteween filenames and the major modes to
load automatically with those filenames.
Re-arranged init.el, added some documentation.

It makes sense to have the smaller customizations at the top of the
file. This way, if something breaks I still have ido mode and backups
are still saved outside of the current directory.

Also, having them at the top makes them more visible, and more likely
to be re-factored if anything gets out of control.