fixes bug 785430 - Add a service to add new products

@rhelmer r?

This basically just does what [the documentation](http://socorro.readthedocs.org/en/latest/installation/add-product-release.html) says to do except it makes it possible to kick this off from the middleware.

Once this has been done and you think it's good, I'll add it to the webapp and its admin UI. 
If any of those fields is missing, it will raise an error that won't be caught. Wouldn't it be better to use ``external_common.parse_arguments`` in this function as well, and handle those mandatory parameters in a more robust way (by raising a ``MissingArgumentError`` for example)?