Messages now display in new version of Ember

Hi Ryan,

This pull request resolves issue #11. 

After looking into things on my local machine, it seems like Ember was not faithfully calling the `activate()` callback. Instead, I've updated **ember-flash-messages** to use the `enter()` callback, which appears to fix everything. I think `enter()` is [technically a private method](https://github.com/emberjs/ember.js/blob/v1.5.0/packages/ember-routing/lib/system/route.js#L43), but it's really just a proxy for `activate()` anyways, so I wouldn't be too worried about using it.

I wish I could understand exactly what was going on, but I'm having trouble understanding the core Route object.

This PR also includes:

1. Bumping the version of Ember to 1.6.0-beta.5
2. Bumping the version of handlebars to 1.3
3. Cleaning and rebuilding the plugin _(my last PR did not do that)_
4. Updating the README to reflect the changes in the last PR