987663 - syncing of a distribution now uses a nectar factory to get the ...

...most appropriate downloader type for a given URL, defaulting to the requests library for HTTP. It also now uses the nectar config options that are specified in the importer config instead of always using a default config.

https://bugzilla.redhat.com/show_bug.cgi?id=987663

Unit tests are forthcoming in another large PR. I don't want to hold up progress on this bug waiting for that whole PR to be done.