fix typo in word Exception
fix imports in geom files
Merge pull request #268 from cosinequanon/fix-geom-imports

fix imports in geom files
Merge pull request #267 from cosinequanon/fix-typo-in-geoms

fix typo in word Exception
Change legend and handle fill and alpha mapping

- legends are no longer added to ggplot object
- fill aesthetic mapping implemented
- alpha aesthetic mapping implemented
- moved legend code from ggplot.py to legend.py
- ':::*_mapping:::' hack to avoid collisions
Merge pull request #271 from has2k1/refactor-legend-fill

Change legend and handle fill and alpha mapping
Initial Separation of stats from the geoms

** Noteworthy activity **
- stats are now in a separate directory
- each call to `geom_abc(stat='bin')` uses the specified `stat`
  this creates an object `geom._stat`.
- `stat.__radd__` creates and adds a `geom` to the ggplot object
- A proper implementation of `stat_bin2d` only constrained
  by the lack of `fill` mapping
- A `geom_histogram` is an alias of `geom_bar` and the bin
  computations are done in `stat_bin`
- `geom_vline` and `geom_hline` can plot multiple lines

** issues **
- `stats` are not computed on proper groups, so the plot may not
  be correct for some factorings.
- Related to the non existent `stat` grouping, any coloring is broken.
- Wrong test cases have been exposed but since the current results are
  also not purely correct the expected results have not been adjusted.
A few fixes from the review
Rename geom._groups to geom._units

It avoids a mix up with the real groups created
by the aesthetic mappings and the group parameter.
Clean up API

- Introduce `geom._extra_requires` to fill the gap between the
  required aesthetic and calculated stuff from the statistics.
- Remove the 'label' parameter where it was used, that is the
  job of gg_title. i.e ggplot level and not geom or layer level.