add examples to rotate x axis labels to quickstart documentation

wrobstroy - thanks for this great library.

I was just playing with Vincent and needed to rotate the x axes labels for some long named items.  After playing around a bit I figured it out and thought I would include a similar example in the docs. Not sure it fits where I put it but I believe it may be useful to people to see how to easily customize one of your pre made Charts.

Another useful example would be mouse-over events, for example on scatterplots. 

zach cp