Changed plotting routines to use PropsSI internally

I have created a proposal to make use of PropsSI method in plotting routines:

* all calculation of fluid properties in plotting routines are now done with PropsSI method
* user inputs in `draw_isolines` and `set_axis_limits` can be done in kSI or SI units (default kSI) and will be converted to SI units for internal use
* all internal plot data will be handled in SI units
  * axis ticks will be scaled to show plot in kSI units
  * if the plot is manipulated by matplotlib API e.g. `axis.set_ylim(min, max)`, then the values has to be in SI units
* changed unittest for plots to use image comparison from [matplotlib testing framwork](http://matplotlib.org/devel/testing.html)
  * plots will be compared against images stored in repo (I have created the baseline images with default matplotlibrc. But I'am not sure if the tests will run on Travis CI properly, because the appearance of the plots could differ depending on the installed fonts on the system.)
  * all tests in `test_plots.py` ran fine before and after change to PropsSI usage (in python 2 and 3)

I think this might be the cleanest solution for the unit handling (convert user inputs to SI; use only SI internally; scale plot for output in other unit systems). What do you think about this?