[Propose] Add a method to remove user from all groups.

From what I understand this has been inquired about before, I propose a removeAllGroups() method be added to the User Model.

```php
	/**
	 * Removed the user from all groups.
	 *
	 * @return bool
	 */
	public function removeAllGroups()
	{
		foreach ($this->getGroups() as $group)
		{
			$this->removeGroup($group);
		}

		return true;
	}
```