Make dtrace-provider optional

Hi,

I've updated the module to make the dtrace-provider dependency optional due to issues with Windows installations. The work around is structured in the same manner as that in node-restify. 

Tests are getting 1145/1145 on my Mac with Node 0.10.24. I can get a test on a Windows machine tomorrow if needed.

Thanks for the module and if anything is incorrect let me know!