Support for multiple tracking objects

Hi there,

I really like your angular module for easy Google Analytics integration. Thank you so much :)

The only thing I'm missing is support for multiple tracking objects. I need this for my project so I decided to make a small pull request including this feature. I'm sorry but I only implemented the 'create' and 'send pageview' events with the corresponding tests.

I'm not sure if I will have time to implement more cases but I thought it is worth to share it with you.