Allow vectors to be plotted on arbitrary map projections.

This PR adds support for plotting vectors on arbitrary map projections. The method used to transform vectors prior to plotting is suitable for any vector field that is defined as grid eastward and grid northward components. This covers all the cases listed in #125. We can even transform vectors from Stereographic to PlateCarree as long as the vector components are defined relative to the grid:

![ps_to_ps](https://f.cloud.github.com/assets/390602/1156470/386cdce2-1f88-11e3-8159-8595dbf514d5.png)
![test](https://f.cloud.github.com/assets/390602/1156471/3a70965a-1f88-11e3-8662-7809a6241503.png)

I've included two mpl integration tests, this is my first major contribution to cartopy so please let me know if I have too few tests or too many tests or the wrong tests or whatever.