Add decimal option for formatting axis labels

This adds a dataAxis.format.left.decimals option for formatting the labels of the axis. It extends the getCurrent member of DataStep. If decimals is undefined, it reverts to the original implementation. Otherwise, it will remove numbers, or add 0's as required to set the decimal places.

One point to note (and the reason I've not added doc) is that for this, and the title option I added in a separate PR, in my opinion this isn't the nicest way to add the options. However, I've effectively extended the way the options are currently implemented (ie ```customRange.left```, and ```customRange.right```). This ends up with a lot of objects, and I think it would be better to change this to ```dataAxis.left.customRange``` etc so that all options for an axis are in a single object.

Just my thoughts though - I'm happy with whatever you prefer... If you're happy with this, I'll add doc.