fix #26: updated bundled deck.js, including the fix on browsercast

this bundle is used also for browsecast-less presentations
and thus browsercast should not unbind the space in this case
and that's solved now

consolidate badge classes to swc.css

Figures should be centered
and not overflow the screen.

the class removed are unused as we take advantage of classes provided by
Bootstrap instead.

Add documentation about card class

Better style for challenges

Fix background for lessons

There's a note in
https://github.com/swcarpentry/lesson-template/pull/182
explaining why we need to fix style for panel heading.

This makes some things look nicer (e.g. adds gradients, etc).