Correct timestamp & pretty print JSON

Timestamp format in demo.js was not in ISO-8601 format. Replaced dash
(-) with 'T' between date and time parts and added a 'Z' at the end to
indicate UTC.

Also, added some decoration to the output and changed JSON to formatted
for easier reading by humans.
Merge pull request #3 from SignalK/pretty-demo

Correct timestamp & pretty print JSON