Increase EFQ query capabilities

When #1138542: Add support for custom Views handlers or something similar is implemented, people may define their own custom view handlers. However, when creating such a handler for a EntityFieldQuery the querying possibilities are limited. Sometimes module developers know for sure that their entities will be stored in an SQL database so why not provide them full SQL capabilities? The following patch lets you execute methods on query objects by registering them to the query metadata system.

Below is an example of how to use this for filtering out NULL values:

/**
 * Custom filter handler for views to filter for, or filer out 'empty' values.
 */
class example_handler_filter_has_value extends views_handler_filter_boolean_operator {
  function query() {
    if (empty($this->value)) {
      $this->query->query->addTag('meta_sql');
      $this->query->query->addMetaData('meta_sql', array('isNull' => array('example_field')));
    }
    else {
      $this->query->query->addTag('meta_sql');
      $this->query->query->addMetaData('meta_sql', array('isNotNull' => array('example_field')));
    }
  }
}

Since it is just a small patch i believe adding it to efq_views will be beter than making it a separate contrib module.