New truncate and truncateWords methods

New truncate and truncateWords methods, https://github.com/yiisoft/yii2/issues/2941
Update BaseStringHelper.php

Length check.
Update CHANGELOG.md
Update BaseStringHelper.php

Updated to use mb_* functions
Test case for new truncate methods.

Test case for new truncate methods.
Update BaseStringHelper.php
Update BaseStringHelper.php
Changelog update
Merge branch 'stringhelper-truncate' of github.com:Alex-Code/yii2 into Alex-Code-stringhelper-truncate

Conflicts:
	framework/CHANGELOG.md
Adjusted phpdoc, tests and changelog for StringHelper::truncate and StringHelper::truncateWords