Fix batman/tools/utils.js and batman/tools/server_utils.js to work with latest Node "path" module

Before this fix I was seeing:

```
path.js:360
        throw new TypeError('Arguments to path.join must be strings');
              ^
TypeError: Arguments to path.join must be strings
    at path.js:360:15
    at Array.filter (native)
    at exports.join (path.js:358:36)
    at Function.exports.send (/usr/lib/node_modules/batman/node_modules/connect/lib/middleware/static.js:141:20)
    at send (/usr/lib/node_modules/batman/tools/utils.js:102:38)
    at /usr/lib/node_modules/batman/tools/utils.js:126:26
    at Object.oncomplete (fs.js:107:15)
```