Upgrade to Node >=0.10 and assorted minor fixes

- Upgraded the code to work on >=0.10: fixed the 'path.existsSync' stuff
- Moved the '-g' option of ffmpeg to be an output option
- Use a proper temporary directory in tests

To be fair, there's nothing in here that stops you running with 0.6 upwards, but the whole self-signed cert stuff means that if you're using 0.6, you should really be upgrading anyway.