Fix starting up the plugin for a second and subsequent times.

Add callback() to else clauses unblocks the startup when the
plugin is running for the second or further times.

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This week of work on @hoodiehq is sponsored by SpotMe.
SpotMe are the inventors of UberApp and a leading brand
in the mobile event app market, constantly working to
improve their software and services to make meetings
even better and make mobile technology a more human
experience. They love offline first, AngularJS, CouchDB,
node.js and the CAP theorem and are now searching for
talented and dedicated people to join their international
team. SpotMe wants you, e.g. Full Stack Developers and
Backend Engineers: http://go.hood.ie/spotme_hires