Add Float._eval_is_infinite

!finite does not imply infinite, so this is needed to make
Float('+inf').is_infinite == True.
Update facts on oo, -oo, and zoo

Only add the facts that are necessary (namely, infinite, and positive and
negative for oo and -oo resp.). The other facts are deduced automatically by
the assumptions system.
Fix a test in test assumptions

!finite does not imply infinite, so the tests were changed to use
infinite=True instead of finite=False (so that the deductions of, e.g.,
(positive=True, infinite=True) + (finite=True) == (positive=True) still
work).