Replace subscribe form inline JavaScript with HTML5 placeholder and allow admin to set placeholder text

Use input type="email" instead of "text"

Input should be empty if !user_email

Add IE9 <label> fallback w/JavaScript

Fix fallback label formatting and add text fallback

Changes per @georgestephanis

Remove superfluous esc_html() parameter from subscriptions.php line 636
Merge pull request #932 from CFXd/improve-subscribe-widget

Replace subscribe form inline JavaScript with HTML5 placeholder and allow admin to set placeholder text