Update push documentation with new example

[crandmck]

The example was recently updated, but the docs were not: http://docs.strongloop.com/display/LB/Push+notifications

Need to update to use the 2.0 server https://github.com/strongloop/loopback-component-push/tree/master/example/server-2.0 and generally clean up and improve.

[crandmck]

I finally updated the doc. It could probably be further improved. Please review and make further suggestions.