Update AG UnifiedPush overview image

An update on this image would be nice:
http://staging.aerogear.org/docs/specs/aerogear-server-push/aerogear_unified_push_server.png

A few things need to be better reflected:

• the "Mobile Variant Instances" are now named "Installations"
• there is no device representing a SimplePush client (e.g. a device and "AeroGear.js" or something like that underneath it)