Convert class to instance methods

This is to avoid weird side-effects when defining filters.
Simplify setting of AbPanel.env
Rename ab_panel_id to distinct_id
Scope Mixpanel keys on Rails environment

This also introduces functionality that skips tracking an event or
user when in a Rails environment that is not defined in mixpanel.yml.
Remove public/index.html from app example
Correctly reset environment

So we don't cache env and conditions.