presenter display: fix notes view on Firefox

Gecko doesn't seem to infer the reduced margin the same way WebKit
does, but explicitly stating the -50% margin works correctly on both
browsers.

Fixes https://github.com/schacon/showoff/issues/167.
presenter css: remove trailing whitespace
Merge pull request #207 from durin42/fix-firefox-presenter

Fix for presenter view on Firefox (issue 167)