Add optional Bootstrap 2 theme

Created by: cvrebert

I got some degree of confirmation about these plans in passing while conversing with from @mdo, but I think it'd be good to start getting the word out publicly so that all the current users who find the new default "plain" look too "flat"-like (* raises hand *) don't up and rage-quit Bootstrap v3. There are also some decisions about the theme that might be useful to get feedback on.

To reiterate why this isn't the default styling for BS v3, let me quote @mdo in a recent issue:

The intention is to enable easier overrides and customization for folks who treat Bootstrap as a first-step, not an end solution.

The choice was based on pragmatic considerations, not flat-vs-skeuomorphic design ideology (as some seem to have misinterpreted the decision). It's simply that fancy gradients make things harder to further customize heavily compared to plain solid colors.

Caveats:

• Just getting "plain" v3.0.0 fully polished and out the door is going to take a lot of effort, so it's possible/likely that this theme might not get made until some time after the release of v3.0.0-final.
• These plans are not finalized. @mdo could change his mind.