Track unstructured events

**work in progress - not meant to be pulled just yet**

Adds `trackUnstructEvent` to the JS tracker and logs the unstructured fields as URL-encoded JSON.

Given that even modern browsers have a URL length limitation of around 2000 characters [1], we were thinking of using Base64 encoding similar to Mixpanel [2] to log `ue_json` with the benefit of being 25% shorter which would allow for more fields. The downside is that it makes this portion of the raw logfile unreadable and unsearchable (so no `grep "foo": "bar" *.log` if that `foo` lives inside the unstructured payload or easy debugging in the Web Inspector).

What do you guys think?

[1] http://stackoverflow.com/questions/417142/what-is-the-maximum-length-of-a-url-in-different-browsers
[2] https://mixpanel.com/docs/api-documentation/http-specification-insert-data

// cc @lackac, @tarsolya