Replacement for @raoulb pr#914

See #914

ToDo:
- [x] tests
- [x] final rebase
On Fri, Jul 18, 2014 at 05:29:06PM -0700, raoulb wrote:
>    In sympy/functions/elementary/trigonometric.py:
> 
>  >
>  > -class asin(Function):
>  > -    """
>  > -    asin(x) -> Returns the arc sine of x (measured in radians)
>  > +    _reciprocal_of = None       # mandatory, to be defined in subclass
> 
>    Should the regular functions carry the same tag too?

Maybe.  But I don't any points to add this now, unless we
change classes to use this attribute.