Add some "tester" methods to JSON abstraction

[tonihele]

Add some "tester" methods to JSON abstraction. Namely these:

• isJsonObject
• isJsonArray
• isJsonPrimitive
• isJsonNull

I can't asses would they break the abstraction or not (i.e. how difficult these are for different JSON implementations to implement). And of course these are not used in jME itself so in that way this is a questionable request. I leave this to your discretion.

Here is our glorious transition from 3.7 beta to 3.7 stable:
https://github.com/jMonkeyEngine/sdk/pull/618/files#diff-2c58043a06f20e6b566c4e3620e809eeeaad2aff34a1679e8afb2ac02a46fedf