Replacement for @raoulb pr#914

See #914

ToDo:
- [x] tests
- [x] final rebase
On Fri, Jul 18, 2014 at 04:55:02PM -0700, raoulb wrote:
>    In sympy/functions/elementary/tests/test_trigonometric.py:
> 
>  > @@ -472,6 +474,13 @@ def test_cot():
>  >      assert cot(11*pi/7) == -cot(3*pi/7)
>  >      assert cot(-11*pi/7) == cot(3*pi/7)
>  >
>  > +    assert cot(x).is_bounded is None
>  > +    assert cot(r).is_bounded is None
>  > +    i = Symbol('i', imaginary=True)
>  > +    assert cot(i).is_bounded is True
>  > +
>  > +    assert cot(x).subs(x, 3*pi) == zoo
>  > +
> 
>    Do we really test all rewrites from any to any other function?

No.

N/B: rewrites tests are in test_sin_rewrite and so on.