Enable external tracker configuration

! Fix beforeUnload for Chromes (this time it's an ugly synchronuous call
  to our server -- further research is required)

  Piwik.js's solution was useless. :(

- Move beforeUnload related code to tracker
- Rename algernonTrap -> AlgernonTrap
- Simplify AlgernonTrap and Transport code
- Make Transport default url the default version: "/v1"
- Add configurable Transport url and headers