use a provider to set some default configuration to all of our popover

For a projet I'm working on, I have to use a lot of popover directive who are almost the same, so it become annoying to have to add the same attributes every time. So I modify the directive to be able to set some default configuration for all my project.

I simply create a ```setDefaults``` method in a provider for the ```nsPopover```, to use it, I simply have to add a config on my project like this : 

```
angular.module('myApp', []).config([
  'nsPopoverProvider',
  function (nsPopoverProvider) {
    return nsPopoverProvider.setDefaults({
      theme: 'ns-popover-tooltip-theme',
      plain: true,
      trigger: 'mouseenter',
      placement: 'bottom',
      timeout: 0
    });
  }
]);
```

we only set the options we want to change, all of them got a default value.