Support KSP version checking.

This is a big update, because we now don't show the user (or allow them to install)
any mods which are not compatible with their version of KSP.

Things to note:

* This almost certainly breaks the old registry format.
* This changes the CKAN spec.

Once this merges, I have an update to CKAN-meta that goes along with it.
You can test this code-base with that repo with:

`./ckan.exe update --repo=https://github.com/pjf/CKAN-meta/archive/new_versions.zip`