Add support for Oscar 0.6 and Django 1.5 & 1.6

* Fix templates still using old-style ``url`` tag
* Make bankcard form compatible with Oscar 0.6 changes
* Add compat module for various method calls in the ``PaymentDetailView``
  mixin and the eway redirect view. This is required because Oscar 0.6
  introduced changes in the API due to the new strategy pricing system.
* Update tests and tox configuration for easier cross-version
  compatibility testing.