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. 
Ah, sorry about that. Was not clear from issue #50. 

I'll modify the PR accordingly. 

What was the use case for this method?