Add template shortcut functions for style and script

Would turn
```php
<?php
\OCP\Util::addScript('news', 'vendor/traceur-runtime/traceur-runtime.min');
\OCP\Util::addScript('news', 'vendor/angular/angular.min');
\OCP\Util::addScript('news', 'vendor/angular-route/angular-route.min');
\OCP\Util::addScript('news', 'vendor/angular-sanitize/angular-sanitize.min');
\OCP\Util::addScript('news', 'vendor/angular-animate/angular-animate.min');
\OCP\Util::addScript('news', 'vendor/momentjs/min/moment-with-locales.min');
\OCP\Util::addScript('news', 'build/app.min');

\OCP\Util::addStyle('news', 'app');
\OCP\Util::addStyle('news', 'navigation');
\OCP\Util::addStyle('news', 'content');
\OCP\Util::addStyle('news', 'settings');
```

into 

```php
<?php
script('news', 'vendor/traceur-runtime/traceur-runtime.min');
script('news', 'vendor/angular/angular.min');
script('news', 'vendor/angular-route/angular-route.min');
script('news', 'vendor/angular-sanitize/angular-sanitize.min');
script('news', 'vendor/angular-animate/angular-animate.min');
script('news', 'vendor/momentjs/min/moment-with-locales.min');
script('news', 'build/app.min');

style('news', 'app');
style('news', 'navigation');
style('news', 'content');
style('news', 'settings');
```
in the News app for instance

@jbtbnl @raghunayyar @DeepDiver1975 @MorrisJobke @georgehrke @LEDfan