Added polar function.

[jensaly]

I used the polar plot demo as a reference, and therefore explicitly defined set_rmax and set_rticks despite them being 1 line different from ylim and yticks respectively. These can be replaced as such.

set_rlabel_position sets the angle of the rticks in degrees. Since this is a method in the axis for a polar projection, I had to import the gcf-function to make it work. Many more axis-methods can be added this way, if prudent. Again, compare to the demo.

Updated the examples and CMakeLists with this.

Closes #130