Merge pull request #252 from has2k1/refactor-layers-like-ggplot2

WIP: Refactor layers like ggplot2

Closes: #221
Closes: #160
bump version
changelog
Update scale_y_continuous.py

Parameter 'labels' can be used to manually designate labels in R.
Merge pull request #264 from Xbar/patch-1

Update scale_y_continuous.py
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.