Update UI.body usage to comply with 0.8.0-rc3 changes

[Trindaz]

From the SO question that spawned this issus[1]:

The problem seems to be caused by UI.body member assignments at https://github.com/tmeasday/meteor-router/blob/v0.6.1/lib/router_client.js#L89-L90 . This breaks changes introduced in Meteor 0.8.0-rc3 https://groups.google.com/forum/#!msg/meteor-talk/oatU1zzcaeQ/54ay-pPlRrwJ .

From the release: "Templates no longer automatically access the helpers of ancestors [11]. If you were declaring helpers with UI.body.helpers you need to switch to UI.registerHelper instead."

Fixing this probably partially fixes fixes issue 140 (Make compatible with Blaze).

Expect a PR from me shortly on this, a fun way to make my first contribution to the meteor community!

[1] http://stackoverflow.com/questions/23232216/why-doesnt-this-template-code-from-the-discover-metor-book-work/23232287#23232287