Adds bootstrap's thumbnail style to figures.

This Pull Request tries to meet the needs of those who use a Markdown and reStructuredText, adding the `.align-left` class, `.align-right`, `.align-center`, `.figure`, `.caption`  and `.legend` classes.

Based in @hlapp's [implementation](https://github.com/DandyDev/pelican-bootstrap3/pull/53), I added a border for `<figure>` and a `.floatcenter` class.