Fixes for CSS double-includes / override behavior

@kylefinley is this roughly what you were looking for in https://github.com/scotch/angular-brunch-seed/issues/91?

It turns out that not only was Bootstrap being included twice in `app.css`, but also the files included in `app.less` (overrides and variables). I moved those includes into a separate directory so that they could easily be ignored (but still watched) without a bunch of extra code in `config.coffee`.

I noticed some differences in margin behavior at >1200px wide; this is because the old version had vanilla Bootstrap re-imported at the end, essentially re-overriding some previous overrides and media queries. Not sure whether you consider that a good or bad thing...