Updates to Highcharts/hPlot

Some updates to the Highcharts class:
- the custom `data` method has been removed
- using toJSON2 to allow for JS functions
- observations with `NA-values` are removed before plotting
- rewrite of hPlot (still not perfect)
- discrete axis labels are now automatically added with hPlot

I think it is unnecessary to keep the `data` method, since most of its purpose has been replaced with `hPlot`. I think (hopefully) that none hasn't really used it yet, so that it's just OK to delete it. What do you think?

For an example, where I make use of JS functions, see [Top 50 DVD Rentals](http://pagist.github.io/?5735146).

@ramnathv I totally agree on your comment about rewriting Highcarts/Rickshaw using message passing to make it more dynamic. Although it could wait a bit, it would be really nice.