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. 
It was going to be used when displaying available apps on the index page, as well as for initialization of those apps. So the method you have right now can stay because we can use it for showing stuff on the index page. Also, your implementation of this can be made better, we don't really need an additional list. You can just iterate over the available apps and build a list dynamically