Updates for issues #72 and #73

[ThomasDaheim]

Pull request for changes to resolve #73 and #72 . Includes updated test case LineChartTest to use the new features.

• added the option to set a StringConverter for number display
• added getValueForDisplay() method as in standard axis
• disabled overwriting of custom tick space values in calcAutoScale() (not a final fix but to avoid overwriting user values til solution)
• matching updates in AxisBuilder
• add style class to TooltipPopup to enable styling via css
• binding the panes bounds to the axis min/max values to update automatically