ggcorr -- a correlation matrix plot with ggplot2

Dear Barret,

I am submitting the `ggcorr` function to your GGally package. It produces a correlation matrix plot that was inspired by [a discussion at Stack Overflow](http://stackoverflow.com/questions/12196756/).

Here's an [up-to-date example](http://rpubs.com/briatte/ggcorr-example).

The function and its documentation are first drafts, and I would be happy to work on both if you think that the function has a chance to make it into your package.

With best regards,

François

![ggcorr](https://f.cloud.github.com/assets/322533/318602/65c4666e-989d-11e2-8aa4-3fbdaeb04aa4.png)