Validate COGs, pixel-space tile serving, niceties, & maintenance

New features:

• Add COG validate helper (see #80)
• Enable * CORS by user choice
• Add is_geospatial property to TileClient
• Add default_zoom property to TileClient to determine starting zoom on map
• Handle tile serving in pixel coordinates for non-geospatial images

Niceties:

• Handle large_image source as input to TileClient
• Add _ipython_display_ to TileClient to quickly display a map in Jupyter
• Add _repr_png_ to TileClient to quickly show a thumbnail in Qt IPython
• Add URL form field to web app

Maintenance:

• Maintenance of Docker images
• Internal refactoring to utilize new server-thread package (see #79)
• Documentation and examples improvements