A few of the discussed updates

[mschottdorf]

Specifically:

• Allows to keep track of when the program was started for the first time; that point in time is stored in the prefs.json file.
• Save the raw meeasurements of flow_out in the hdf5-files, and estimate&subtract baseline in the main loop (as discussed).
• Added try/catch pattern to main.py - Tom and I have been using a script in the sandbox featuring this pattern, but this should be part of the main program.
• Some cleanup in the controller.py
• "Fixed" travis, by figuring out which tests failed. The culprits were two test-blocks in the UI-test, where values.SENSOR.items() and values.CONTROL.items() were checked. I've commented these out, as big UI update is imminent

[mschottdorf]

I'll merge this, so that J can continue with the UI