Upgrade to 3.1.1 and use new file extension

Uses the new `.import.less` file extension supported by Meteor 0.7.7.1 (updates `README.md` and establishes `bootstrap.import.less` and `/lib/less/bootstrap.import.less` to reflect this) . Otherwise, just pulls in the Bootstrap 3.1.1 release's `/dist/js/bootstrap.js`, `/dist/fonts/*`, and `/less/*` files.