feat: add extra logs as option after finish

Added a new option in the `browserStack` config to log more info about the total execution / net time, and the browsers execution percentage over total to give a last recap of everything.

Still need some feedback about the formatting : 

![xgk96](https://cloud.githubusercontent.com/assets/6033345/4430866/6b43d4fa-464b-11e4-8777-de7d2eef9825.png)

And some usages:

   - Is the option name `extraLogs` good 
   - I've used chalk to colorize the output, but it's not mandatory
   - Since it's some recap logs, I've not used the `logger` but a basic `console.info`, but I can go back with this