Added some new steps to test middleman applications

Today you can only test for the absense of a string in output, but not a block. I added a step for that.

```bash
# exists
I should not see "string"

# does not exist
I should not see:
"""
"""
```

Furthermore I need to check for the http status code. There is not step for that. I added a step for this as well.
```bash
the status code should be "200"
```


Do you think those steps are general enough to be helpful for others?

Cheers,
dg