Added interface and basic class for postsale handling
Payment methods must not implement processPostsale
Expercash and Postfinance implement postsale without parent class
CS
Isotope\Model\Payment\Postsale already implements the interface
Most payment methods can use the generic Postsale parent class

It will automatically show a "waiting" message while the postsale
request is received
Added .htaccess to allow postsale script
Added useful http headers for invalid postsale requests
CS
Check if payment or shipping method implement IsotopePostsale interface
Merge branch 'feature/postsale-interface' into develop