fix #13 use tmpdir as cwd for spawned children

When cwd is not explicitly set, and the module is required
from a third-party code, then spawned children used the cwd
of the requester module:

Say I created a code like this (myfile.js):

```
var launcher = require('browser-launcher2');

launcher.detect(function(available) {
    console.log('Available browsers:');
    console.dir(available);
});
```

With undefined cwd, this creates `TempWmicBatchFile.bat` next to
`myfile.js` instead of next to the module's files.

Let's instead use OS-provided tmpdir and don't care about deleting
the temp files on our side.
Or better yet, create a `tmpdir` variable at the top.