Add tax inclusive field to products

Another requirement from my clients – add their products to the system with tax already included.  

Here's what it looks like (the GST in the screenshot comes from the name of the tax in settings):

![price-tax-included](https://cloud.githubusercontent.com/assets/749593/4878422/93ddee14-630b-11e4-96d6-9701031e4a90.png)

As always, there's room for improvement (e.g. introducing AJAX to calculate the value "on the fly" without submitting the form, refactoring this out to be a concern on the model so it can be applied to other models as well), but it's a start.

Also I won't be offended if you guys decide this shouldn't be in the main repo as it stands.  But here for discussion.