Implemented a baseline geom_tile.

The tile geom now creates a tiled geom similar to R ggplot2 with
some caveats:

- If you add the fill parameter to the ggplot aes wrapper, ggplot
  will arbitrarily map colors without taking into account ordering
  of the fill values, which can't (as far as I know) be stopped
  by the geom coming down the pipe. If sent this way, the geom gets
  the already mapped fill values and is unable to recover their
  numeric values to make a heat map image, so all geom_tile plots
  drawn like this are deemed 'categorical'.

- If you add the fill parameter to geom_tile's aes wrapper, the
  actuall fill values are sent, so geom_tile can use them to make
  a heatmap. This is the only way to do that.

So as far as useability is concerned:
  - If you want to tile categorical data, you must put the fill in
    the ggplot aes.
  - If you want to make a heat map, you must put the fill in the
    geom aes.

(Technically one can get around the first restriction by duplicating
code in the geom, but I chose to issue an Exception instead to avoid
duplicating code).
Merge pull request #378 from anthonytw/master

Implemented a baseline geom_tile.