ggcorr -- draw a correlation plot matrix with ggplot2

Inspired by a Stack Overflow question
<http://stackoverflow.com/q/12196756/635806>. Would be very happy to
improve the code.
erge branch 'master' of github.com:briatte/ggally into briatte-master
removed first plot to speed up testing / pass checks
documented
added ggcorr from Francois Briatte.  Added '...' to feed into geom_text.  (had to remove the default of size = 4)