Use Dir.tmpdir in place of hardcoded /var/tmp for state files

On non-windows systems, /var/tmp is used as a hard-coded path for the default state file. Use Dir.tmpdir instead. The path can still be overridden using #UUID::state_file=
Merge pull request #31 from langhorst/master

For issue #30, use Dir.tmpdir instead of hardcoded /var/tmp path