support for geom_point as an alternative to geom_tile

Hi Barret,

## `ggtable`: one-line bar plots for survey responses

The `ggtable` function is a simple wrapper to produce one-way or two-way bar plots out of tables in a single line of code:

![](https://github.com/briatte/ggtable/raw/master/example8.png)

I have roxygenized the documentation to show the several several examples in the `ggtable` documentation.

## Small update to `ggcorr`

The `ggcorr` function can now plot proportionally scaled points in lieu of tiles:

![](https://raw.github.com/briatte/ggcorr/master/example2.png)

The examples have been updated.