BUG: use of pcolormesh silently drops and misallocates gates

As the docs for matpltolib's pcolor/pcolormesh note, pcolor will silently drop the outermost row and column of a 2D grid when passed X and Y whose dimensions exactly match the grid. pcolor needs to know where to put the corners of the grid cells, so the right way to do this is to pass X and Y with dimensions one larger than the grid itself. 

The edges of the gates aren't available, so we have to approximate them by interpolation and extrapolation. Here I provide one version of doing so.

The included images result from running quick_plot_cfradial_rhi.py and quick_plot_cfradial_ppi.py with the changes in this pull request. Comparison with the baseline images in master shows an extra radial of data and an extra gate at range for each radial, and a half-delta shift in the position of the gates and rays.

![example_cfradial_ppi](https://cloud.githubusercontent.com/assets/1325771/3682551/06f04a6e-12d0-11e4-9f1d-a8e255ffc450.png)
![example_cfradial_rhi](https://cloud.githubusercontent.com/assets/1325771/3682552/06f58326-12d0-11e4-99dd-d54ac1b4d01b.png)

I only dealt with the pcolormesh calls in RadarDisplay, so if there are others they should be looked at, too.