Added sech and csch under ReciprocalHyperbolic class

Definitions of csch, asech and acsch will follow in further commits.
On Wed, Dec 10, 2014 at 05:47:49AM -0800, Shivam Vats wrote:
>    In [1]sympy/functions/elementary/tests/test_hyperbolic.py:
> 
>  > @@ -467,7 +603,7 @@ def test_acoth_series():
>  >  def test_inverses():
>  >      x = Symbol('x')
>  >      assert sinh(x).inverse() == asinh
>  > -    raises(AttributeError, lambda: cosh(x).inverse())
> 
>    But we do for tan, cot, sinh, tanh etc. Has the 'inverse' attribute been
>    deprecated?

No.  But these functions are different (i.e. from sin or cosh).