Added new instrument "issuecount"

Displays issue counts. 

Colors can be configured in `teamwall.json`. 
Data format must be `{ "issueCountsOrderedByPrio" : [0, 2, 0, 11] }`

![issuecount_show_colors](https://f.cloud.github.com/assets/128227/158891/751c439a-76f9-11e2-9d85-3d649c001fbc.png)  <span/>  ![issuecount_show_green](https://f.cloud.github.com/assets/128227/158892/7531b748-76f9-11e2-8b50-ea25ae2394c5.png)

An issue count of zero results in a green background automatically.