Add support for more Karma launchers

[lvh]

Currently only Firefox is supported because that was the only one that was easy to get in Travis. However, we should really also run the tests in Safari, Chrome and maybe even Opera. This is well-supported by Karma; we just need to wrangle

• Add appropriate npm Karma plugins
• Add Travis support for testing with Safari on OS X
• Figure out which platform it's easiest to get Chrome and Opera on (it may be Travis' OS X builders) and run it there