BACKWARDS INCOMPATIBLE: Added AJAX compatibility / Allowing to skip confirmation

- field renamed from payerID to PayerID, which is what PayPal returns. You
  might need to update your templates
- Added redirect parameter to SetExpressCheckoutFormMixin - this enables you
  to use this form with AJAX calls as it will only return the redirect URL
  instead of a HttpResponseRedirect
- Added skip_confirmation attribtue to DoExpressCheckoutView. If you set this
  to true, the confirmation view will not be displayed, instead it's POST
  handler will be executed, just as if the user has clicked the "Confirm"
  button. This allows you to save one click during the checkout process.
Released v0.6