PList Store: Find way to handle NSNull values

the "problem" is that the 'null' (from JSON) is, in Cocoa/ObjC, mapped to NSNull. However, NSNull is, unfortunately, not a "valid" PList object/type.

There are ways how we could fix it on the library:

• exclude every key, where value is NSNull (which makes the value 'nil' (not NSNull)
• see if new 'store' could be written, that simply uses NSJSONSerialization, to store JSON on the "filesystem"