Merge branch 'pull-request/257' into PHP-5.5

Merge branch 'pull-request/257' into PHP-5.5

* pull-request/257:
  array_column: Fix compile-time warnings
  array_column: Removed array_pluck() alias
  array_column: Set array_pluck as an alias for array_column
  array_column: Implement ability to specify an index column
  Cleaning up a memory leak.
  array_column: Adding test for IS_OBJECT and converting object to string
  array_column: Using add_next_index_zval() at nikic's recommendation.
  array_column: Improved tests
  array_column: Cleaning up, as recommended in pull request #56 comments
  Fixing typo in test for array_column()
  Simplify the code and use zend_hash_next_index_insert()
  Adding test for columns not present in all rows for array_column().
  Adding tests for the negative results of array_column()
  Implement new array function array_column()