[WIP] Added the definition of sech

Definitions of csch, asech and acsch will follow in further commits.
On Thu, Nov 13, 2014 at 12:13:11PM -0800, Aaron Meurer wrote:
>    Is it actually necessary to define _sage_ if the Sage name is the same (I
>    don't know)? If so, we ought to fix that.

Yes, but can refactor this part by moving this stuff (unless Sage
name *is* different) to the parent classes (probably, to the Function
class).