Allow loading remote Akoma Ntoso files
Make it easier to override the ImportCommand behavior that is filesystem-specific
Don't explode if an image 404s
Even greater realism in mocking a 404 error
Always use force_text, present since 1.4.2.
Merge remote-tracking branch 'james/404'
Tweak test setup so collectstatic isn't necessary.
Add links and testing instructions to README.

Thanks to James McKinney for the suggestion.
Mock urlopen in importer_tests
Merge remote-tracking branch 'james/import'
Merge remote-tracking branch 'james/loadremote'
Fix merge