Factory functions for ingredient and recipe to allow 3rd party extension

[vanbo]

I am building a cooking site with the plugin and since I need to extend the Ingredient and Recipe classes, so that I can add my own methods and functionality, I wanted to see if you are interested in reviewing some pull requests and adding the functionality to the plugin. Since I will be using the plugin to build a complete site I may add some new features and if you are open to it I can push the code to you as well.

I gotta say that I really liked the plugin and even though the not so smart rating somebody gave you, the code is solid and very easy to work with and extend, a nice blend between EDD and WC practices. It far exceeds the other plugin in customization possibilities. I only wish that you used taxonomy for the Ingredient type instead of the CPT, but I can work with that, too.