Added get_non_essential_apps() method.

Addresses #50 

Added essential field to application base class. Wrote registry method to return non-essential apps.

Made files PEP8 compliant.

Tested calling this method, only notes application was returned as expected. 
I think it could be argued either way. For now I'll leave as double since this is the way shared_state is handled. Consumers should call non_essential_apps to get this.