Regridding in projection space for vector plots.

This PR adds support for doing a transform and regrid for vector fields (and scalar fields) so that the resulting points are on a regular grid in projection space. The motivation for doing this is to be able to make nicer looking vector plots, but it is important to note that this PR lays the groundwork for an implementation of `streamplot` in cartopy. 

A new module is added which handles the transform and regrid operations, and a new keyword is added to `GeoAxes.quiver` that allows the user to control regridding (works in pretty much the same way as the `regrid_shape` argument to `GeoAxes.imshow`).

Below are two plots that demonstrate the change, the second one is much clearer:

![vec_normal_color](https://f.cloud.github.com/assets/390602/1500722/772f8aa0-487e-11e3-8ae8-0450d90f24bb.png)
![vec_auto_linear_color](https://f.cloud.github.com/assets/390602/1500723/7ac70cf6-487e-11e3-8833-10462fcbbcf6.png)