Refactoring SMIL parse/generate and adding unit tests.

I have made some changes I would hope to see land with your stuff for SMIL parsing in mozilla-b2g/gaia#8512.

First off, from what I understand about `mozMobileMessage` it should be a complete replacement for `mozSMS` and still provide all the `mozSMS` functionality.  We should probably switch the internal references in the sms app over to this.  If this is not the case, we can remove the commit that does that.

> From @vicamo
> Yes, we're going to integrate MMS and SMS into mozMobileMessage.
> However, we still keeps mozSms (at least for now), and operations
> through either interface should have exactly the same effects and results.

Secondly, at Bocoup we are currently trying to refactor the sms app to be less reliant on the message_manager code at the moment, so I refactored your SMIL stuff into it's own object and added some unit tests.

This could use a lot more unit testing showing more real world SMIL parse/generate scenarios, @steveck-chung if you could help add some more tests here it would be great.  Also, I have yet to have an opportunity to test this on a device with airtime.