New data type - raw_string

Hi,
I added new useful data type for salmon checks - raw_string.
With this data type it's easy to check commands output and compare them with expected results.
I use this feature as http_check. For example:

```yaml
'web*':
    cmd.run "curl -s -I http://localhost/|grep HTTP":
        name: "/ HTTP Code"
        type: raw_string
        assert: "'{value}' == 'HTTP/1.1 200 OK'"
```

I have couple of ideas for extending salmon user panel. I forked the project and I plan to send useful parts of code as pullrequests.