Fix relative pathing issue on multiple installs

[tomlagier]

Currently we're using plugins_url() without a plugin qualifier and assuming that Wordpress installs the plugin under the directory pluginName. This isn't always the case - sometimes it doesn't detect that the plugin is a duplicate and reinstalls it in a new directory, pluginName-X where X is the number of times it's been reinstalled.

If we move calls to plugins_url() to use __FILE__ as an argument, it should fix the issue.