Feature/stat_ellipse

Following up on people saying they like `stat_ellipse`, (http://stackoverflow.com/questions/2397097/how-can-a-data-ellipse-be-superimposed-on-a-ggplot2-scatterplot and http://stats.stackexchange.com/questions/38117/getting-different-results-when-plotting-95-ci-ellipses-with-ggplot-or-the-ellip) here I've properly implemented and documented it for incorporation into the main package. 

It seems to be operating properly, and it passes `R CMD check`.