Allow industry sector tags to be assigned to artefacts

This adds a new select element to the artefact form, so that industry sector tags can be assigned to artefacts. 

I've also added a new helper method which returns options for a select box grouped by parent tags, in a structure that Rails' option tag helpers can turn into `<optgroup>` tags.

The new field looks like this:
![screen shot 2014-01-23 at 14 04 54](https://f.cloud.github.com/assets/71922/1985015/676e3ea0-8437-11e3-8e62-e9dcfef289fa.png)