Add staging support.

[cscatolini]

This PR introduces an isProduction flag to allow switching between production and staging xmpp addresses.

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[cscatolini]

I signed it!

[googlebot]

CLAs look good, thanks!

[appleboy]

This change will break this package which anybody uses currently. You need to create another function to change staging or production mode.

[cscatolini]

@appleboy you are absolutely correct, I've sent another commit fixing this issue.