Move the dashboard tables to django-tables2

Before you merge this I'd port over the other tables as well. This is primarily for review. A few things:

* Getting the sorting icon into the header is a little bit hacky, with an `<i class="icon-">` to trigger the necessary CSS rules for FontAwesome. Also the unicode character value for the chevrons up and down had to be copied. If FontAwesome had less mixins we could use that.
* Sorting and tab are kept when the title link is used (this was a bug in the original product list)
* You can sort by UPC, Product class, status or parent as well

Other than that I'm impressed with the `django-tables2` source code. The repository doesn't look very active, but the code is very high quality from what I've seen today.

Let's see what travis says :)

Advantages to the current approach to tables:
* If customizers want to remove or add columns dynamically in the code, they don't have to do that in two places (table header and table data in the template), but just in one.
* Higher level of expressiveness. Compare what you'd have to do to add another sortable column to the recently_edited tab. The header would have to duplicate the same `if` check as for title.
* Easier customizable: If you want to add or remove columns, you have to copy the whole table template even if you don't change most of the columns.
Dependencies in ``setup.py`` get installed when doing ``pip install django-oscar``. ``requirements.txt`` only gets read when fetching the repo and calling ``make install``.