Add ability to use different variable type for ngModel

Usage:
Default:
```
<toggle-switch ng-model="switchStatus1"><toggle-switch>
```

For number:
```
<toggle-switch ng-model="switchStatus2" on-value="1" off-value="2" on-label="1" off-label="2"><toggle-switch>
```

For string:
```
<toggle-switch ng-model="switchStatus3" on-value="O" off-value="C" on-label="Open" off-label="Close"><toggle-switch>
```