Use anonymous sorting function in ArrayHelper::sortObjects
Add private constructor to disallow instantiation of ArrayHelper
Fix code style
General improvements to ArrayHelper (type hinting, scope, passing by value)

- Add type hinting to array argument in toObject, toString, getColumn,
  invert, isAssociative, pivot, sortObjects, getValue
- Pass variables by value, not by reference (in methods without side
  effects applied to the arguments. So toInteger is an exception)
- toString's array argument is required, not default null
- Make arrayFromObject private (no use having protected methods in a final
  class)
Replace foreach with array_map in trivial cases
Change toInteger behavior

Now returns the converted array (instead of returning void and converting
the argument passed as a reference.)

- Change test to reflect new behavior
- Change Framework usage (in Joomla\Form\Form)
Removing @package [ci skip] Closes #77
Update documentation for toInteger
Merge branch 'master' into improve-array-helper

Conflicts:
	vendor/Joomla/Form/Form.php
	vendor/Joomla/Utilities/Tests/ArrayHelperTest.php
Code style cleaning

Fixing up (mostly) @since alignment.
Merge branch 'improve-array-helper' of https://github.com/eirslett/joomla-framework into improve-array-helper