Call untrailingslashit on plugin dir path and plugin dir url

The functions `plugin_dir_path()` and `plugin_dir_url()` both add their own
trailing slashes. Currently the plugin builds a lot of links with double
slashes in them, like
`siteurl.com/wp-content/plugins/janrain-capture//stylesheet.css` because its
building links by adding another slash ast the end of the output of these
functions.

I don't know if this actually breaks in any common web servers, but it's 
annoying and should probably be fixed.