Moved many dependencies to extras_require

This is an attempt to resolve #218.

It's not ready to merge, as I didn't really know what to do about the site option for Callout.  I did something silly for now.  I created a minimalistic install of widgy[0] in order to test this out.  I wasn't sure how to go about doing it otherwise.  It has a couple of branches for the different dependency groups.

I also removed the dependency on South.

[0]: https://github.com/fusionbox/widgy-bare
When would WIDGY_PAGE_BUILDER_SITE ever be used as a setting?