Remove dependency on 'Origin' header.

The 'Origin' header is not always provided when calling getPluginPath (in firefox, but also chrome when doing a GET request). This causes the plugin path to be wrongly constructed and loading of stylesheets/images fails. It now constructs the origin in the javascript and appends the relative plugin path.
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Merge pull request #11 from thehyve/issues/fix_firefox_genome_browser

Issues/fix firefox genome browser