Delete sensors.

- Add API endpoint to delete sensor (doesn't delete measurements)
- Add delete button to Edit Device UI
- Remove alert banner when new view is loaded (which previously was displayed throughout different views unless it was closed by the user).

Fixes #386 

![image](https://cloud.githubusercontent.com/assets/2522010/4847226/1935293c-6051-11e4-9a0a-c72a485a2500.png)