Rewrite/rename import_reg management command

[cmc333333]

In the process, drop workarounds for Python 2.6 (we support 2.7, 3.4+), remove
unnecessary code, and add tests.

Builds on #64

[coveralls]

Coverage increased (+9.5%) to 92.955% when pulling 99c963b on cmc333333:rewrite-import into e19380d on eregs:master.

[coveralls]

Coverage increased (+9.5%) to 92.955% when pulling 014e481 on cmc333333:rewrite-import into e19380d on eregs:master.

[gregoryfoster]

Thanks for taking care of this, @cmc333333. I'd suggest also updating the project's README.md to update usage of the command.

[cmc333333]

@gregoryfoster thanks! I think the latest commit does that, but maybe I missed something?

[gregoryfoster]

@cmc333333, my mistake! I missed that file in a scan of the changed files on the pull request. Confirmed locally by looking directly at the file.

[cmc333333]

Whoot, whoot, thanks for reading it over!

[coveralls]

Coverage increased (+0.009%) to 92.955% when pulling 6384a89 on cmc333333:rewrite-import into de7e36f on eregs:master.