Create ad4julia.jl

This module declares a dual number type for Julia. See http://en.wikipedia.org/wiki/Dual_number. These are useful for forward-mode automatic diffentiation.

This implementation allows generalizations by allowing the dual number components to be of type 'Number', rather than just type 'Real'. This allows one to nest dual numbers recursively, or to use complex components. It does not allow vector or matrix-valued components.