`netkan` no longer requires a KSP install to operate.

For a cache, we now use the following:

- What the user specifies, via `--cachedir=...`
- The local CKAN cache, if we can find it.
- The tempdir for the machine.

This means that the `netkan` utility can be used on machines without KSP or the CKAN client installed.

Closes #190.