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)
`type: :feature` and `js: true` have been specified in the describe. They should not specified here.

It's also more appropriate to use `scenario` instead of `it` for feature specs.