Converting CSS to SASS

This PR represents my attempt to convert the existing stylesheet set to SASS as per #917.

So far, I've kept the original filenames to ensure maximum compatibility with existing layouts. Later on, we'll be able to group stylesheets by functionality using some sort of purpose-specific map; say, app.css for most application-specific functionality, editor.css for editor styles, and so on. Once we start to go with a mapped approach, we'll be able to adapt that to other SASS frameworks if we decide not to continue with sinatra-assetpack.

The only internal stylesheet I haven't converted to SASS yet is the file viewer styles, because it doesn't seem to match the main Gollum style set at all.

I've also made a few changes which are just provisional; I'm happy to revert them if you feel like they're not desired.

* Changes conditional comment for IE7 stylesheet to apply to IE8 as well
* Global font stacks have been consolidated down to one regular set and one monospaced set
* Searchbar styles tweaked to look more consistent on mobile
I think this mixin can be omitted [nowadays](http://caniuse.com/#feat=border-radius).