Add ability to define sandbox or production environment at class construction

Hi,

I had se same question as the one in https://groups.google.com/forum/#!searchin/easyapns/development/easyapns/ZZBH2b3scts/uYpOF5jDIuQJ
Then I added environment support to you class.
So if you think that passing environment at class construction is ok this way, please merge.
Don't hesitate to comment this commit if you want changes to be done.

Thanks,
Nicolas.