use parameter to store bootstrap location and also configure to allow ov...

...erriding of bootstrap location from theme. (allows customization of bootstrap via LESS via http://getbootstrap.com/customize/ and usage without changing core files)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -