Unpinned the Cordova plugins, including the File* ones

Mostly this has required changing from using `fileEntry.fullPath` to `fileEntry.toURL()`

Updates were needed for the FileViewerPlugin as well. These are currently in a branch called "update-to-3.5.x". This branch of the plugin is needed to test these changes.
Re-pin the plugins at their current version.

... so that they don't change until we're ready to change our cordova
version.
Remove superfluous comma.
Merge pull request #566 from SpiderOak/issue-472-update-cordova

Issue 472: Update Cordova