Use Bootstrap for examples

This PR:

 * Uses Twitter Bootstrap for the examples
 * Adds a "simple" example that can be used as a base for new examples

Still to be done:

 * conversion of the `example-list.html` page to Bootstrap. 
 * fix error:

```
2013-03-05 16:09:58,127 check-examples: phantomjs bin/check-example.js build/gh-pages/HEAD/examples/anchored-elements.html?mode=whitespace
JavaScript ERROR: INVALID_STATE_ERR: DOM Exception 11: An attempt was made to use an object that is not, or is no longer, usable.
TRACE:
 -> file:///home/travis/build/twpayne/ol3/build/gh-pages/HEAD/build/ol-whitespace.js: 14594
```