people.unset() and miscellaneous fixes

The library now supports people.unset(); see https://mixpanel.com/help/reference/javascript-full-api-reference#mixpanel.people.unset for documentation.

NB: If using the embed code "snippet" for your mixpanel installation, please upgrade to the latest snippet to use people.unset() safely.

Other fixes:

• Identify Chrome OS correctly (for $os property)
• Bugfix: register_once() no longer overwrites previously-registered but falsey values
• Stub parts of DOM API when loading in non-browser environments (thanks @gfx)
• Log full "Implementation error" exceptions when DEBUG mode is on