Update KerbalStuff Transformer

[dbent]

This makes a few small changes to the KerbalstuffTransformer:

• Licenses we receive from KS are trimmed, I've seen a couple of cases of otherwise valid license strings being rejected because they had leading or trailing whitespace.
• The specific cases of GPLv2 and GPLv3 are transformed into CKAN compatible license strings, since those are two of the default options for licenses on KS.
• The x_screenshot property is moved from being a top-level property to a child of resources, which I think makes more sense.

[dbent]

@plague006 I think that comment was meant for #1304? Done in 23dbb4a.

[mheguy]

Yes it was. Deleted my comment because it's off-topic.