Adds a "Test drive" code example to the homepage, updates the mission statement, and updates notes on BEM

The test drive stuff on the homepage is to help new visitors "get it" by allowing them to play with cf components in an easy and test-it-out kind of way.

I'm trying something out with the mission statement. Per the OSCON workshop a mission statement should "emphasize the promise, opportunity, and definition of what your community is trying to achieve." So I figured I'd flesh ours out a little.
I'd like to clarify this sentence a bit. How does this sound?

"Adding an element to a modified block can result in a confusing class name like `.list__space_item`. Avoid this in favor of using a descendant, like this: `.list__spaced .list_item`.