Add OC\VObject\StringProperty. Partial fix for owncloud/apps#762

This fixes a flaw in Sabre\VObject\Property where you couldn't escape commas and semi-colons in properties as required by the rfc, because the serializer would then double escape it.
As VObject doesn't (yet) have a deserialize() method, it is up to the implementer to to unescape using somthing like `$value = strtr($property->value, array('\,' => ',', '\;' => ';', '\\\\' => '\\'));`

Edit: Or we could patch OC_VObject::getAsString() to do this.

To enable this for an app, add the properties you want to use this class to Sabre\VObject\Property::$classMap in appinfo/app.php e.g.:

```
Sabre\VObject\Property::$classMap['SUMMARY'] = 'OC\VObject\StringProperty';
Sabre\VObject\Property::$classMap['DESCRIPTION'] = 'OC\VObject\StringProperty';
```

@bartv2 @georgehrke 

Refs owncloud/apps#762