Migrate to Bootstrap 3 - fixes #11

This was not just a simple upgrade from version 2 to 3. I have preserved the look from Bootstrap 2. For some elements like the search field, I had to copy the css from Bootstrap 2 into the Asimov less files.

Some mixins have disappeared and with them some background gradients. I'm going to test this on our test server at Tradera before merging this in. I will do some more clean up later and remove unused css and images.