split __repr__ into draw and __repr__ and set interactive=False in draw()

`draw()` only draws the plot without showing it, `__repr__()` then uses
plt.show() to open the plot (which only does it if interactive == True
and a suitable backend is used). This change is needed for saving a
plot via `ggsave(plot)`.

To make this possible, interactive = True has to be set to False
while in `draw()`. Set interactive=True for the rest
(= `__repr__()`) during importing and based on `sys.flags.interactive`.
Add a fullfeatured ggsave command

Mostly it is feature complete to the one from ggplot2, including
filenames, sizes and making it possible to save a specific plot.
TST: Don't test images other than png

The test default to test png, pdf and svg images, but this
a) doesn't work on stock windows and
b) is probably irrelevant as we don't want to test backends.

It also was already disabled for some tests.

So disable every extension but png.
TST: Remove now unneeded baseline images (pdf, svg)
TST: tool to show all visual tests in the browser
TST: Update scale_reverse test und baseline images

The test images are still not perfect (-> legends) but the main functionality
is working and looking good in the tests.
Implemented scale_facet_wrap function.

Added support for calculating the limits and setting the label visibility for
four different scale types when the facet type is "wrap": None, "free",
"free_x", and "free_y". The behavior of the different scale types is based on
the behavior outlined in the ggplot2 docs
(http://docs.ggplot2.org/current/facet_wrap.html), except I decided to display
the x-axis labels on the bottom plot for each column, not just the plots in
the bottom row.
Renamed xdim and ydim to rows and columns, respectively.

Makes the code in the scale function easier to understand. Also added a note
to the call to scale_facet_wrap about how n_wide is actually being used to
specify the number of rows in the subplot and n_high is used to specify the
columns.
Tests for the scale_facet_wrap function
Forgot to commit this to add the scale_facet_wrap test to the test suite.
Add more programmatic testing to the scale_facet_wrap test.

Don't just rely on the image comparison, also run some checks by inspecting
the axes of the generated plots and making sure that ticks and labels are
present and equal where appropriate.
inc version; spacing
removing strange facet label thing
Merge pull request #107 from darthmall/scale_facet_wrap

scale_face_wrap implementation