enable & disable bar, event Listeners

added a variable for disabling/enabling the bar, default is of course enabled. The reason for this is, that maybe you just want to use the awesomeness of the bundled xhr events without the bar, only the spinner. Just do 
```javascript
angular.module('myApp', ['chieffancypants.loadingBar'])
  .config(function(cfpLoadingBarProvider) {
    cfpLoadingBarProvider.includeBar= false;
  })
```

added events for loading started and completed, so that you can do stuff in both cases, such as hide elements or the like. You can listen via 
```
$rootScope.$on('cfpLoadingBarCompleted')
```
and 
```
$rootScope.$on('cfpLoadingBarStarted')
```