Improved taxon admin view

I improved the taxon view in the admin and added a functionality to remove a product from a taxon using the API. Added tests as well.

To be more clear, some screenshots:
Before:
![screen shot 2014-12-26 at 15 00 07](https://cloud.githubusercontent.com/assets/106335/5558108/d3195bda-8d19-11e4-9746-f50b8fab670d.png)

After:
![after](https://cloud.githubusercontent.com/assets/106335/5558109/de03ad70-8d19-11e4-8c46-6ea2e4db6376.png)
I would prefer that this use some form of capybara-like selector rather than using execute_script. The [Capybara Select2 Helper Gem](https://github.com/goodwill/capybara-select2) can be helpful with that.