Preliminary (and backwards-compatible) Meteor UI support

This is a small patch which makes iron-router work with Meteor UI (without breaking support for Spark).

Ultimately, a lot of things can be implemented quite differently now.  This patch doesn't attempt to change the current underlying mechanisms, and indeed, function names and comments in the code refer solely to the Spark approach.  The main idea was a minimal patch to add backwards compatible support for Meteor UI, so that people can check out the preview release and still have routing support, and not break iron-router for existing releases.

Checked with:
```bash
mrt --release template-engine-preview-1
mrt --release 0.6.6.2
```

Don't forget to change {{loginButtons}} and {{yield}} to {{> loginButtons}} and {{> yield}}, respectively.  I still need to look at named yields, but this is working great for my purposes so far and I think is a good starting point for now.