Test for plotting beachballs in non-scaled axes

Test contains 2 cases: x larger than y; y larger than x. Both have
reference images. Both are still failing.
Plot Beach circular in non-scaled axis

Added an `axes` kwarg to `obspy.imaging.beachball.Beach` that, when
given, will maintain the aspect ratio of the beach ball on non-scaled
axis. The ratio is maintained even when resizing the plot.

Inspired on the code for matplotlib.pyplot.scatter.
Test using width,height in Beach non-scaled axis

Passing an axes and (width, height) with height=0.5*width to Beach
should make ellipses.
Test Beach aspect ratio using 1 beach only

Reduced the tests for Beach aspect ratio in non-scaled axes to a single
beachball, not a collection. As suggested by @mbyt.
Merge pull request #734 from leouieda/beachball-aspect-ratio

Maintain beach ball aspect ratio