Add support for pie charts

I've implemented support for pie charts.
While doing this, I've also fixed the fact that the "show legend" option didn't do anything (the legend's containing div wasn't written to the HTML output)

I didn't quite understand how the "flot" style was supposed to work, and I've done some tweaking so it works for me when having a view which shows two fields; the first being the x-axis and the second being the data point (Y-value). If this is wrong somehow, please let me know how it's *supposed* to work.

There were also a few inconsistencies in how arrays and objects were treated which I've fixed.

To make pie charts work at all, I moved all options for bars, lines and pie into a "series" sub-option. The top "lines" and "bars" options are just there for convenience I think, and they're copied into series anyway. However, pie charts don't do this so it breaks when you don't have them below "series".

If you want to follow along with development, you can also clone http://dev.solide-ict.nl/git-pub/flot.git and pull from there...