#47 symlog scaling

[Nico Schlömer]

matplotlib supports three scalings for axes:
* linear,
* log, and
* symlog.
see http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.xscale.

linear and log are supported by pgfplots, symlog isnt'. Please add it.

(The idea for this is from https://github.com/nschloe/matplotlib2tikz/issues/38).

[Stefan Pinnow]

For the record:
Here is a question that also wants this feature on TeX.SX:
- http://tex.stackexchange.com/questions/348998
(and marsupilam provided a solution which might serve as a good start for an implementation?)

And this I moved from the PGFPlotsToDo here:
http://stackoverflow.com/questions/3305865