Possibility to choose test (folder) name instead of current datetime

Add extra parameter to the run command with unique test name. I use this to create URL for the Jenkins Plot plugin: /sitespeed-result/%index%/index.html
```
./bin/sitespeed.io -u http://www.sitespeed.io/ -i build1

Folder: sitespeed.io\sitespeed-result\www.sitespeed.io\build1\index.html
```
![sitespeed unique url example in jekins](https://cloud.githubusercontent.com/assets/657797/4353157/fcf0f30e-422e-11e4-9b75-24a71f7bb801.png)