Coding standards FTW

* Apply WordPress code standards
* Introduce WP_STRIPE_URL constant which contains the url to the plugins directory
* WP_STRIPE_PATH now returns a path as you would expect
* Dynamically get the plugin version rather than defining it manually
* Pass the plugin version as the version param when registering scripts and styles
* Escape dem JS variables
Merge branch 'master' of github.com:humanmade/wp-stripe