setPlatform support

[rcs]

Adds support for setPlatform in the javascript tracker. Example use case here would be html+js running inside node-webkit, or a webview on a mobile platform.

[alexanderdean]

Nice! Thanks Ryan. Scheduling for 0.13.0...

[rcs]

This raises an issue re: https://github.com/snowplow/snowplow/wiki/snowplow-tracker-protocol#2-platform-specific-parameters . There we're saying those things are only feasible for the "web" platform, but all the rest of the "html-js medium" platforms would make sense to have those values too.

[alexanderdean]

Yes - we have to be a bit careful here. We need to decide if we want setPlatform to be an informational thing or to proscribe what parameters are available/expected on this platform.

A webview on a mobile platform is a good example of this: our current thinking is that the platform should still be "web", because all of the context available to a web browser is available to this event. But this isn't very informational if you know this tracker will only be used on mobile devices...

/cc @yalisassoon