Created a Set Complement class

Sergey B Kirpichev <skirpichev@gmail.com> helped with some fixes
in the PR's https://github.com/hargup/sympy/pull/5 and
https://github.com/hargup/sympy/pull/4

* Remove duplicated UniversalSet class
* Reenable some old tests
* Fix random test failure

Conflicts:
	sympy/sets/tests/test_sets.py
Write printers for Complement
Merge pull request #7462 from hargup/set_difference

Created a Set Difference class