Add method to enable/disable the observers calls

Suppose that we have to insert some rows to category table. So we do this:

```
// Getting the category table
$category = JTable::getInstance('Category', 'JTable', array('dbo' => $this->_db));

// Bind data to save category
if (!$category->bind($row)) {
	throw new Exception($category->getError());
}

// Insert the category
if (!$category->store()) {
	throw new Exception($category->getError());
}
```
But this way to insert a new category returns the error: 'Application Instantiation Error' produced by the observers calls attached by default. Maybe this is an issue for the observers, i dont know why honesty, but i can solve the problem commenting this line:

https://github.com/joomla/joomla-cms/blob/staging/libraries/joomla/observer/updater.php#L104

The problem is that there is no way to disable the observable calls, so this patch add the method to disable the observable calls if the developers not need it.