Updates readme.

**Still a super rough draft**

Here are some updates to the readme.

The changes:

1. Explains Marionette more succinctly and descriptively (I hope)
2. Removes unnecessary stuff.

Things removed either:

1. Don't belong anywhere
2. Belong on the website

While we might not be able to merge it just yet, I'd like to see us work toward getting something like this in.
Heh, the slogan is already there. I want this to be a sentence that's a bit longer than the slogan, but describes what Marionette is. The second below starts with things that Backbone lacks, which starts us off on a negative foot. I want to begin positively by saying what Marionette can do for you, then go into the details of why you *need* something on top of Backbone.