Added signal level and valid altitude to JSON-feed
Added 'validaltitude' and 'signal' to JSON-feed.

Signal is array of last 8 signal levels where [messages % 7] is latest signal level.

If planes register is supplied with JSON it is linked to planespotters.net in detailed table.

	modified:   dump1090.c
	modified:   public_html/planeObject.js
	modified:   public_html/script.js