Added relationship resolution logic.

Part of #32.

Looks like it's mostly working. Will have to remove some diagnostics
from here, and have discovered bugs in the installer which need fixing.
Fix ModuleInstaller to make moar directories.

Because apparently we can't trust zipfiles to contain them
reliably.
Added CKAN.User.YesNo() for prompting users.
Prompt the user before installing a million mods.
Removed the world's least informative debugging statement.
Registry.LatestAvailable() now throws ModuleNotFoundException
Added cmdline options to control relationships on install.
Merge pull request #67 from pjf/32_relationships

Implements much of #32 (relationship handling).