Cache the KSP gamedir in the registry.

Relates to GH #28.

Currently we just do this when we find the gamedir for the first
time. There's no way for the user to specify it.
Added --ksp= switch to override KSP directory on cmdline.

For #28.
Added `ckan config gamedir` to set the gamedir.

Closes #28.
Use IsKspDir() for --ksp switch.
Merge pull request #55 from KSP-CKAN/registry_28

Adds KSP directory setting, both cmdline and registry.