Update to make dtrace-provider an optional dependency

Hi Mark, 

I'm trying to get node-ldapjs working on Windows without the need to compile the dtrace-provider dependencies and as this module is a dependency of node-ldapjs I have modified it to make dtrace-provider optional. 

I haven't modified the test script to run in a Windows environment but all tests are passing on OSX with dtrace-provider installed and also without.

Thanks, 
Evan