Capture partial payments for the wirecard payments

@aprofeit @BlakeMesdag  @jduff please review.
/cc @ntalbott. I saw your pull request, it looks good but it was missing a few things (outline below). I already had this completed, so I figured I'd just push it.

This pull request to solve issue: https://github.com/Shopify/active_merchant/pull/560

* Added add_amount function to add amount for capture and transaction
* Changed authorize method to use the wirecard preauthorization api
  * Partial captures don't work with 'authorize'
  * Preauthorize allocates funds for the merchant
* Changed capture_authorization action to just capture
  * capture_authorization is an invalid function for partial captures
* Added remote tests and unit tests
  * Also changed the mocked response to reflect changes in api functions
  used
* Changed permitted_transactions to reflect changes