Release Name: 0.5.0

Notes:
Tiny graphs.


Changes:
== 0.5.0

* Documentation for bullet graph.
* More bullet options for good_color, satisfactory_color, and bad_color.
* Smooth options for target line value and target_color.
* Sparklinks.plot_to_image returns a Magick::Image object for further manipulation.
* Step option for wider whisker.