Specify the default aesthetics for the geoms

**stuff done**
- geom.DEFAULT_AES a dict with the default values for the
  aesthetics
- geom.plot_layer to handle per geom manual aesthetics and
  have them override those in ggplot.
- Save test result images that are more representative of
  of the desired outcome.
- Add a missing __future__ import for py3 compatibility.
- Removed ggplot._get_layers, I had forgotten about it.