ENH: reverse_xaxis parameter added to plot_rhi

RadarDisplay object's plot_rhi method now supports a reverse_xaxis
parameter when will change the east-west direction of the plot to a
west-east direction.  The default of None reverses the direction only
when only negative distances are present.

Closes #111
Merge pull request #113 from jjhelmus/master

Enhancements and bug fixes based on issues