Deactivate the email notification on a per check basis.

refs #13. Added a way to deactivate the email notification on a per check level.

This pull request allows user to deactivate the email notification on a per check basis.

The example below works but [yaml](http://yaml.org/type/bool.html) allows some more variations.
```
"*":                        # Salt target                                      
    service.status nginx:               # Salt function(s) to run against target
        type: boolean                   # Expected result type                 
        assert: "{value} == True"       # Fail is this evals to False          
        alert_emails: False
```

or 

```
"*":                        # Salt target                                      
    service.status nginx:               # Salt function(s) to run against target
        type: boolean                   # Expected result type                 
        assert: "{value} == True"       # Fail is this evals to False          
        alert_emails:
```