Add support for multiple y-axes

This PR adds support for multiple y axes:

  - yaxis is configurable in the frontend using "axis" parameter
  - each channel configured as "auto" is assigned to an axis with matching unit
  - if no axis with matching unit exists, a new y-axis is added to the right side of the chart
  - channels can be forced to display on a particular axis if unit matches or not (this is the current behaviour)

WARNING: this PR needs some testing. While the functionality should work it may not be desirable for all constellations. Do we e.g. need a setting to turn it off?

![unbenannt](https://cloud.githubusercontent.com/assets/184815/2792342/2966c6ea-cbd1-11e3-8271-ea037a564be5.png)

PS.: also corrected some minor code formatting issues and changed the day names to something ore sensible.