MinMaxBase will raise error if args are not Expr's

This also fix deprecation warnings like this:
```
>>> s = FiniteSet(FiniteSet(1, 2, 3))
>>> s
{{1, 2, 3}}
>>> len(s)
1
>>> s.powerset()
/usr/lib/python3.3/site-packages/sympy-0.7.5_git-py3.3.egg/sympy/functions/elementary/miscellaneous.py:334:
SymPyDeprecationWarning:

is_real has been deprecated since SymPy 0.7.6. Use is_subset(Reals)
instead. See https://github.com/sympy/sympy/issues/6212 for more info.
```