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. 
@naturalwarren: having a list becomes a problem when we have a lot of registered applications. There's general convention to make things that need to be iterated over into generators rather than build a full list and keep it around. It's the same reason we use xrange instead of range. Generators are usually preferred over straight lists and iteratables.