Pytest-fxa

* Match session when verifying account and cleanup when verification fails.

* Allow environment(s) to be specified using a custom ``fxa_env`` marker.

* Catch the exception in teardown when the account has already been destroyed.

* Provide a ``fxa_email`` fixture for accessing the email address.

* Allow users to specify the email address by passing ``--fxa-email`` command
line option or setting the ``FXA_EMAIL`` environment variable.

* Initial release