Added finish-unfinish-error url (landing page)

[rizdaprasetya]

• new function landing_redir() to handle redirection from VTWeb
• new function failure(), to handle failure payment
• relocated cart->empty, cart is emptied when customer reach payment success page after redirected from VTWeb
• locally tested, should be running as expected

  TODO

• update docs.veritrans.co.id for new finish-error-unfinish url which is :

http://[your shop’s homepage]/index.php?route=payment/veritrans/landing_redir&