Added sech and csch under ReciprocalHyperbolic class

Definitions of csch, asech and acsch will follow in further commits.
On Wed, Dec 10, 2014 at 04:36:37AM -0800, Shivam Vats wrote:
>  > +    assert sech(1) == sech(1)
> 
>    Such tests exist for other hyperbolic functions.

It's your code, your responsibility.  And you can't introduce
such tests just by copy & paste.  So, the question still same.