active_merchant: allow dispatch of requests to different accounts

By adding a list of credentials in your yml configuration, e.g.:

:my_gateway:
  - :account_id: :credentials_1
    :login: admin_1
    :password: password_1
  - :account_id: :credentials_2
    :login: admin_2
    :password: password_2

you can now specify which account to use for a given payment transaction
by adding an entry for payment_processor_account_ids (csv) or payment_processor_account_id
in the options parameter.

Note that split settlements (ability to dispatch one transaction to multiple accounts)
is not fully implemented yet (see remaining TODOs).

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>