Escape strings passed to share widget properly

Calling `htmlspecialchars()` on this text more-or-less does the job of
preventing XSS attacks, but it makes the titles and descriptions that
are passed to the share widget have ugly entities like `&#8217;` instead
of single quotes.

`esc_js()` handles replacing invalid UTF8 or dangerous entities, as well as
adding slashes before single quotes. Also, `esc_url()` ensures that only
valid URLs are shared.
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.
[DEV] update default screens to work with new Plugins flow!
Make the $capture object a public global

Currently all global variables are completely encapsulated, so there's
no way for developers of other plugins to hook into any of this plugin's
actions and modify them.

This goes against the WordPress way of exposing filters so that plugins
can build on each other.

With this change, for example, it's possible for a theme to remove the
Capture scripts from views such as modal windows that don't need them,
by doing something (admittedly clumsy) like this:

    global $capture;
    $capture_ui =& $capture->ui;
    remove_action( 'wp_head', array( $capture_ui, 'head' ) );
    remove_action( 'wp_head', array( $capture_ui, 'sign_in_screen_js' ) );
    remove_action( 'wp_enqueue_scripts', array( $capture_ui, 'register_scripts' ) );
    remove_action( 'wp_footer', array( $capture_ui, 'share_js' ) );
    remove_action( 'wp_footer', array( $capture_ui, 'sign_in_screen' ) );
    wp_dequeue_style( 'janrain_share' );

With the current plugin, there's no way to do this at all, or to
override the settings passed to the widget.
Re-add the default styles

Where did these go?  here they are!
updated verify screens
updated signin screens
updated public profile screens
update forgot password screens
update edit profile screens
Merge pull request #11 from thanj/than/dont-escape-share-title

[EXT-288] Escape strings passed to share widget properly
Merge pull request #12 from thanj/thanj/untrailingslashit

[EXT-288] Call untrailingslashit on plugin dir path and plugin dir url
Merge pull request #13 from thanj/than/public-entry-point-for-hooks

[EXT-288] Make the $capture object a public global
[DEV] Clean-up
Merge branch 'Screen-updates' into dev

Conflicts:
	janrain-capture/janrain-capture-screens/edit-profile.html
	janrain-capture/janrain-capture-screens/forgot.html
[EXT-304] removed recaptcha option and hard-coded it into the widget JS settings
Merge pull request #15 from janrain/EXT-304-Recaptcha-Removal

[EXT-304] removed recaptcha option and hard-coded it into the widget JS ...
[EXT-323] Fixed loadjs path construction
Merge pull request #16 from janrain/EXT-323-Widget-Migration

[EXT-323] Fixed loadjs path construction
[EXT-339] added backplane server base url to the admin page

[EXT-339] added backplane server base url to the admin page