Implementation of the stroke_axis helper method

In issue #296 it was mentioned that the #stroke_axis helper from the manual should be implemented directly in Prawn.

This pull request contains an implementation of the #stroke_axis helper including method documentation. The manual has also been updated to use the now built-in method. I have also added some more options to customize the output.

Specs are not included because I don't know how to correctly test this new feature.