Add partial captures to wirecard gateway
* 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
Removed a comment that no longer applied.
Merge branch 'master' of github.com:Shopify/active_merchant into rlb-wirecard_partial_capture
An attempt to fix failing tests on the Ci server caused by whitespace.
Update the CHANGELOG
Merge pull request #571 from Shopify/rlb-wirecard_partial_capture

Capture partial payments for the wirecard payments