World mercator

Improving the Mercator Projection in Cartopy CRS.

The prior Mercator Projection was not correctly specified in terms of proj4 params or how it was coded. This PR thus replaces the Mercator class, using the correct proj4 params and other improved code, so bringing the code style more in line with other projection classes.

Altering the Mercator class has caused secondary impacts within the Cartopy code, notably in web image tiles code (for example Google Tiles) as the projection used for such web maps is often Mercator. Similarly, a number of test images and tuples have been altered to reflect the change to the Mercator Projection.