Templatetag for static and some refactoring.

What I did:

- templatetag to render jQuery and/or Highcharts javascript files (can be overriden by a setting)
- some refactoring: essential is done in HighChartsBaseView (general chart options and series) and HighChartsDualAxisView (xAxis/yAxis), while specific views does not include yet specific treatments (I used almost every class and didn't need yet some specific option)
- some changes on variable names: since the project has just been initiated, I didn't see a real choice in the mapping between class attributes and Highcharts options names. Thus, I decided to keep it very simple: attributes have the same name and must match Highcharts structure if overriden in your classes
- get rid of "y_axis_title option" since it is possible to have multiple Y-axis and the way it was done didn't match with this need (title can now be set by creating y_axis attribute, with Highcharts structure).

I think the project still needs improvements/discussion before telling the world, but for me and some easy charts, it did the trick!