enabled wrapping for pcolormesh when using Mercator

When plotting global data defined on a 0-360 lat-lon grid with pcolormesh onto a Mercator projection, cartopy is not wrapping the data as expected. 
![figure_1](https://cloud.githubusercontent.com/assets/1634089/3312644/fa68c174-f6d5-11e3-93a9-1d25aee7b417.png)

This PR fixes the issue.