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.
(cherry picked from commit
6d191d78dfafd07daeb3b39e140677c10a1e4a6a)