Replacement for @raoulb pr#914

See #914

ToDo:
- [x] tests
- [x] final rebase
On Fri, Jul 18, 2014 at 05:20:28PM -0700, raoulb wrote:
>    In sympy/functions/elementary/trigonometric.py:
> 
>  > @@ -1125,7 +1002,7 @@ def fdiff(self, argindex=1):
>  >
>  >      def inverse(self, argindex=1):
>  >          """
>  > -        Returns the inverse of this function.
>  > +        Return the inverse of this function.
> 
>    Why do not all functions have this inverse function?

I don't know.  Now added for asec/acsc.

>    The one of tan has the same spell error.

Actually, I think it's your spelling error.  This
change in cot was reverted.