Add $prefix in SASS to allow scoping of CSS about messenger

[zackbloom]

Makes sense to me, it will also be necessary to add the ability to pass that prefix into the javascript, so it can reference the classes appropriately.

[mrchess]

I guess .messenger is really the prefix already by design. I ended up replacing all messenger in the CSS into a $prefix, and for JS I added a baseCSSPrefix global which has the same prefix the swapped out messenger with #{baseCSSPrefix} in the templates -- similar to Ext 4s method for their Sandbox Mode. (http://dev.sencha.com/deploy/ext-4.0.0/overview/index.html)

[zackbloom]

PR?

[mrchess]

I'll send one up soon. I realized I wasn't sure if it is a good idea to
merge it in since in a way it adds some overhead as you meed to type
#{prefix} all the time which can get kind of annoying in practice.