added helper for displaying tags as labels

I played around a little bit with the Kitematic source. 
This is not a perfect solution, if you don't like the changes code-wise, please take it as a feature request.

I just thought, that KM could display the tag information in a more condensed way.
It may also be more suitable if not the helper extracts the tags, but some other function beforehand, but I didn't dive too deep into the code.

Anyway, here's how it would look like:

![bildschirmfoto 2014-12-28 um 01 11 31](https://cloud.githubusercontent.com/assets/649031/5562660/04b375fc-8e2f-11e4-98b5-70467662b35d.png)

![bildschirmfoto 2014-12-28 um 01 07 46](https://cloud.githubusercontent.com/assets/649031/5562659/04b34c08-8e2f-11e4-9ad1-e8ebe23a9cf2.png)