Plugin cleanup

Created a repo for the plugin and gitignored the plugins folder.

This means that a fresh clone needs to have the plugin added. I have added instructions for this to the README.

When the cordova cli tools support it, upgrading the plugin when it changes should be easier, but for now, it involves removing the plugin using `cordova plugin remove FileViewerPlugin` then re-adding it using `cordova plugin add /path/to/FileViewerPlugin`