Created a Set Difference class
Removed S.Universal Set from the set logic
Wiped out UniversalSet class
Rescructed UniversalSet from previous commits

Conflicts:
	sympy/core/tests/test_args.py
Updated docstring for complement
Fixed the doctest and removed unecessary comments
explicitly state args/kwargs for Difference
Resolve import error for Difference

Conflicts:
	sympy/sets/fancysets.py
updated test_args for Difference
test_inequalities: updated for set difference
Allowed difference for finiteset with symbols
Fix complement tests in test_inequalites
Remove now irrelvent TODOs [skip ci]
sets: universal_set -> universe [skip ci]
remove trailing whitespace
Remove evaluate=False from set.__sub__
Renamed Difference -> Complement
fixed illfixed from the rebase
changed __sub__ -> _complement
Added reference to Complement in the docs