Link bill to event

Added a dropdown to link a bill to an event. Only list started events (once https://github.com/my-koop/module.event/pull/12 is completed)
Currently not showing any more information about that, but will be useful to generate reports

![image](https://cloud.githubusercontent.com/assets/4157103/5272438/fb102010-7a4a-11e4-9aa7-f741f4af1e92.png)

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no, I simply don't use this variable directly it will always get assigned before