make it possible to use multiple to_email in XML

The Configuration class allows to configure more than one recipient.
Since the to_email field was configured with XML configurations as
an attribute couldn't be specified multiple times.
bug #91 make it possible to use multiple to_email in XML (xabbuh)

This PR was merged into the 2.1.x-dev branch.

Discussion
----------

make it possible to use multiple to_email in XML

The Configuration class allows to configure more than one recipient (see symfony/symfony-docs#4135). Since the ``to_email`` field was configured as an attribute in XML configurations, it couldn't be specified multiple times.

Commits
-------

44505b6 make it possible to use multiple to_email in XML