Initial refactoring of ECGView

Now an application wide listening on the devices is setup; the ECGView
then accesses this device-data-buffer, which is constantly updted
throughout the runtime of the application. This ensures that the ECGView
doesn't redraw from new data, when it is shown --> now previously
fetched data is redrawn too.

Conflicts:
	src/controllers/device_controller.js
	src/templates/studies/show.handlebars
	src/views/ecg_view.js
Initial go at using app wide buffer
Change from setTimeout to promise when waiting for devices
Minor refactoring

* Increase defaults for secondsToShow
* Since the values for the Y axis are constant over time, the call to
* update the axis domain can be safely removed
Add filtering for ECG values

The filter checks for negative values smaller than -1.0mV and for values
which' change to the next value is more than 1mV