getlantern/lantern#1974 Added timeout to NAT-PMP logic

Closes getlantern/lantern#1974
Hmm, now that you mention it ...

The test always ran fine on my Mac, but I had to make some tweaks to the testing code to get it to run on Windows and Ubuntu.  I've now tested on OS X, Windows XP 32-bit, Windows 8.1 64 bit, Ubuntu 14.10 32-bit and Ubuntu 14.10 64-bit.

If you ever want to build cross-platform tests, you can use our fork of [gox](https://github.com/getlantern/gox) to build unit test executables by supplying the -test flag, like this:

```bash
gox -osarch="linux/386 linux/amd64 windows/386 darwin/amd64" -test
```