Y-axis labels: hidden overflow fix


I know there is a (y)axisLabelWidth option, but in our application we don't know what for data the user will load into the dygraph, so we can't really put a fixed value on this.

Therefor it would be better if dygraph would always fully show the values in the Y-axis, no matter what the axisLabelWidth might be. Otherwise it can create situations like this:
![wp_ss_20130209_0002](https://f.cloud.github.com/assets/1446260/191787/af00bd54-7f49-11e2-9d56-bea267e7c6ee.jpg)
In this graph the amount of handles of a particular process is shown, but it should be 60000 handles instead of 600. Also in our application some values have a special value and are being replaced by text, as you can see in the graph at the bottom, where it should say 'Not initialized' instead of 'Not'.

In this patch I also added a backgroundcolor option, so when the labels would overlap with the axis/graph the label would have a background, just like the mouse over legend overlay.