Update pelican-bootstrap3 to the latest version

Hi,

There have been a lot of changes in to [pelican-bootstrap3](https://github.com/DandyDev/pelican-bootstrap3) since I last merged to _pelican-themes_. The theme has become [quite popular too](https://www.google.nl/#q=pelican-bootstrap3) (pages of blogs "powered by pelican-bootstrap3"). 

One problem I encounter is that the theme gets updated quite often and fast, which forces me to merge all those changes into _pelican-themes_ as well, all the time. I remember one of the _Pelican_ folks once saying that they would rather not include a theme as a submodule for some reason, but in this case it really seems an attractive option! I already had some bug reports in the source repository based on an older version of _pelican-bootstrap3_ that is included in _pelican-themes_. Is there something we can do about that?