aerogear-ios-push Documentation update on code signing

Update documentation with:

1. code signing
As burr suggested it, it might not be clear : "how does someone "selected this certificate inside of xcode”?"

Let's add a “Code Signing Identity” and “Provisioning Profile” explantion and screenshot.

2. Profiles migration
What about adding a section “Profiles Migration’ at the end of our iOS tutorial here[1]
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Christos
[1] http://aerogear.org/docs/unifiedpush/aerogear-push-ios/provisioning-profiles/
[1] https://developer.apple.com/library/ios/recipes/xcode_help-accounts_preferences/articles/export_signing_assets.html#//apple_ref/doc/uid/TP40013306-CH8-SW1