Have a fallback ready if require('os').tmpdir isn't available.

This adds node.js 0.8 compatibility.

After updating node-pngquant to node-pngquant-bin 0.1.7 compatibility with node.js 0.8.x was lost because it doesn't have `os.tmpdir()`. I'd really like to stay compatible with 0.8.x for at least a little longer, so please consider merging this.

I copied the implementation from my own module that does pretty much the same: https://github.com/papandreou/node-gettemporaryfilepath :)