Replacement for @raoulb pr#914

See #914

ToDo:
- [x] tests
- [x] final rebase
On Fri, Jul 18, 2014 at 05:27:54PM -0700, raoulb wrote:
>    In sympy/functions/elementary/trigonometric.py:
> 
>  > @@ -1114,7 +959,39 @@ def _sage_(self):
>  >
>  >  class cot(TrigonometricFunction):
> 
>    cot is also a ReciprocalTrigonometricFunction, at least mathematically.
>    Why is this implemented differently?

Perhaps, historically.  It may be not easy to reimplement
this as a ReciprocalTrigonometricFunction.  In this case - I
will create an issue, ok?