Fancify README with Markdown, build status, links

Transition README into Markdown formatting, still persisting the
ability to read it as plain-text, asides from the status badges.

Add AppVeyor and Travis CI status badges to the top of the page.  Not
only does this make it easier to access build history, it's a pretty
quick way to tell if the build is working or not.

Add a few more handy links to (hopefully) help those stumbling across
Quassel's dev repo without prior knowledge.

Closes GH-209.