Draw the grid on the whole figure

This fixes #72. 
I hope I have chosen the right approach to solve this issue: when computing the grid lines, I replace the use of the wcs '"footprint" (0, 0, nx, ny) by the size of the figure axes (`self.ax.xaxis.get_view_interval`). 
I can remove the last commit (pep8 cleaning) if you find it easier to review. 

Example:
https://f.cloud.github.com/assets/311639/513889/8ddd758c-be4a-11e2-9cb6-20b2cd1c9593.jpg