Merge pull request #77 from jjhelmus/master

ENH: RSL no longer a required dependency
BUG: lat, lon incorrect when using grid_origin.

When a grid was created using map_to_grid the lat and lon keys in the
axes attribute did not reflect the correct location of the grid origin.

Created a test to detect this bug and corrected it.
Merge pull request #78 from jjhelmus/master

ENH: GridMapDisplay: a class for creating basemap plots from Grid objects

Closes #67
Merge pull request #79 from jjhelmus/grid_origin_bug

BUG: lat, lon incorrect when using grid_origin.