Make dtrace-provider an optional dependency

Hi Mark,

Similar to my pull request to your node-pooling module I'd also like to make dtrace-provider optional in this module. The tests are passing on OSX with dtrace-provider installed and without it.

Naturally this change would add the requirement to update the versions of the modules for npm and have this module depend on the new version of node-pooling to fully remove the dtrace-dependencies. I'd be very grateful if you could do this.

Thanks,
Evan