Node < 0.10 compatibility

Using Node < 0.10 (test with Node v0.8.2):
```
var tmpdir = require('os').tmpdir();
TypeError: Object #<Object> has no method 'tmpdir'
```
Node.js v0.10: [`os.tmpdir()`](http://nodejs.org/api/os.html#os_os_tmpdir)
Node.js < v0.8: [`os.tmpDir()`](http://nodejs.org/docs/v0.8.0/api/os.html#os_os_tmpdir)