Added set($key, $value) method on repository

As for now, if you want to set resource's value you should do something like this:
```php
$resource = new Resource();
$resource->setKey($key);
$resource->setTextValue($value);
$manager->persist($resource);
$manager->flush();
```
So you must know if the resource exists and what is the type of resource (field which holds its value).

This PR simplifies this into:
```php
$resourceRepository->set($key, $value);
```
This is a BC break and can't be merged into 1.0 branch.