atomic-emacs class for all key bindings, toggle command, startup pref

[zot]

Resubmitting this pull request after I configured my email address for git on my new laptop :).

Hi Evan (er.. George, I guess?) -- I made atomic-emacs bindings able to be toggled with shift-ctrl-space so that people can easily access key bindings in other packages that might conflict with atomic-emacs. I did this by making sure .atomic-emacs is in every key binding (except shift-ctrl-space).

Also, I made atomic-emacs auto-start and install bindings by default and I added a setting to control whether to do that.

Finally, I added a couple lines to Readme.md about shift-ctrl-space and the startup setting.