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.
Merge pull request #12 from thanj/thanj/untrailingslashit

[EXT-288] Call untrailingslashit on plugin dir path and plugin dir url