Remove unnecessary native dependency

It makes this nice API wrapper unusable in [projects that have windows support](https://bitcointalk.org/index.php?topic=209149.msg5027777#msg5027777) (without requiring all users to go through hell first).
Merge pull request #5 from askmike/patch-1

Thanks! Didn't realize microtime wasn't supported on Windows. I suppose connection speeds will bottleneck it to far greater than 1ms, anyway. :)