Added fill_srtm and shade_srtm to srtm.py
osgeo is needed for filling the SRTM gaps.
Added an example to show how to get and shade an SRTM relief map
Added gdal in travis-ci configuration.
Merge pull request #396 from pelson/srtm_shading

Elevation filling & shading by @ThomasLecocq.
Added __getstate__ attr to Img class to aid pickling
added utm crs
Merge pull request #399 from marqh/utm

Added the Universal Transverse Mercator CRS.
Review actions: pickle round trip test now separate
Merge pull request #402 from dkillick/pickle_img_nest

Added __getstate__ attr to cartopy.io.img_nest.Img class
Update test image for new Google tile label style.
Merge pull request #408 from rhattersley/tile-update

Update test image for new Google tile label style.
Add WMTSArtist and GeoAxes.add_wmts().
adding Mizurski's Terrain for the US
Add owslib to .travis.yml.
Fix OGC module name in test.
Adjust imports.
Combine tile images to avoid visible seams.
Clamp tile indices and clarify epsilon usage.
Fix desired pixel span and clip final image.
Merge pull request #409 from rhattersley/wmts-artist

Add WMTSArtist and GeoAxes.add_wmts().
Update to latest Google tile style.