Use MessageChannel to support Chrome <51, test with localtunnel.me for https

[nolanlawson]

Fixes #7

MessageChannels are preferred in the case of Service Workers, because event.source is null in Chrome < 51 (bug), which means you can receive messages but can't send any back. This can be revisited once Chrome <51 drops to low usage numbers and we can just switch to event.source, but for now this works in Chrome 42+ and in Firefox. For Web Workers, I stick with the old system and avoid unnecessary MessageChannel creation.

I also needed some way of actually testing Service Workers to prevent regressions, meaning I needed Zuul to run over HTTPS. I decided to go with localtunnel.me because ngrok, although it also offered https, was too restrictive with the concurrent number of tunnels. I tested manually and confirmed that this is actually testing HTTPS and actually testing Service Worker.

Unfortunately I had to set the tested Chrome versions to 42+ because, although Service Worker is in 40 and 41, it has some bug in MessageChannels that breaks the test (no time to debug to figure out what the issue was). Luckily Chrome 40 and 41 have very low usage.

[nolanlawson]

Background: #7 #8

[coveralls]

Changes Unknown when pulling c60ba24 on 7 into * on master*.

[nolanlawson]

Sweeeet, finally passing