Upgrade to bootstrap3

This is an incomplete PR attempt for #178, which is upgrading bootstrap from v2 to v3.

I'm unsure which files from the current version of dillinger have been modified, but it seems the new navbar width is much smaller.

Overall the app looks "okay" but it doesn't look like it is before using bootstrap v2. I would suggest that if we want to change to make it more closely resemble what it looks like now, we put all changes into `custom.styl`. However I'm having trouble getting the styling to work without appending `!important` to the styling, which seems to be a bit of a hack.

Anyways. Not sure which direction to go into, just making a PR to show that we can at least attempt this fully.