Use unsigned ints internally so value of ifaceIdx matches kDNSServiceInterfaceIndexLocalOnly

I noticed a problem with some of the unit tests on my machine (Mac OS 10.8.5), kDNSServiceInterfaceIndexLocalOnly had the value 4294967295 instead of -1 which was causing some of them to fail.

This pull request applies the changes from [this commit](https://github.com/Wizcorp/node_mdns/commit/c5098d9d812aaf5b3a4f622122aeb817cd6484ab) to all relevant operations.

There's also a change to the Browser class to test for the local-only interface first as it's no longer -1 so won't trigger the `&& ifaceIdx > 0` bit of the if clause.