Add WMTSArtist and GeoAxes.add_wmts().
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().