Update to latest Google tile style.
Merge pull request #414 from rhattersley/update-google-tiles

Update to latest Google tile style.
Fix missing matrix set name in tile request.
WMTS support for EPSG:900913 (Google Mercator).
Move defn of CRS to cartopy.crs.GOOGLE_MERCATOR.
Merge pull request #411 from rhattersley/google-crs

WMTS support for EPSG:900913 (Google Mercator).
Merge pull request #413 from rhattersley/matrix-set-name

Fix missing matrix set name in tile request.
Simple support for limited area WMTS matrices.

Just ignores any TileOutOfRange errors.
Added wms to GeoAxes.
Merge pull request #412 from rhattersley/simple-out-of-bounds

Simple support for limited area WMTS matrices.
Implements WMS as a RasterFetcher and adds the SlipperyImageArtist for
RasterFetcher interactive visualisation.
Clear image accumulator to avoid ghosting.
Merge pull request #416 from rhattersley/wmts-ghosts

Clear image accumulator to avoid ghosting.
Updated documentation of get_extent.
Merge pull request #418 from pelson/get_extent_doc

Updated documentation of get_extent.
Review actions.
Renamed SRC.
Merge pull request #415 from pelson/wms_artist

GeoAxes.add_wms()
Port WMTS rendering to GeoAxes.add_raster().
Update docs and WMS tests.