Adds option to exclude validators for monitors in configuration.

Validators can be excluded by adding the "validators" property to a monitor and listing the names of the validators to be excluded in an array called "excludes"inside inside of "validators".

Example:
```javascript
"numenta/experiments": {
    "username": "numenta-ci",
    "password": "CHANGE ME",
    "contributors": "http://numenta.org/resources/contributors.csv",
    "validators": {
        "excludes": [
            "Travis Validator"
        ]
    }
}
```