Modernize the build system

[KrisSiegel]

In continuation of the modernization effort in #71, the build system in use by msngr.js is quite ancient. Seriously, the bulk of it was written in early 2013. It's not bad that it survived for so long! But, it's kinda clunky and difficult to deal with (there isn't even a dev server monitoring for changes!).

So, I have a list I'd like to see tackled here

• Explore replacing grunt with a newer build tool if it provides enough utility over straight up writing scripts (which honestly isn't half bad but handy things like tree shaking are sorely missed). I'm thinking rollup.js here considering this is a library.
• Explore setting up a development server that monitors for changes and builds as you go. Optionally runs tests each time it rebuilds.
• Explore better browser version testing. Right now tests get tested in a single version of chrome and that's it. It would be fantastic to be able to test against multiple browsers and multiple versions.