maintenance release

Merge pull request #1768 from vetalt/shouldHandleOnlyZeroInSlug

nullable slug must not be null if base field has "0"

maintenance release

Merge pull request #1728 from Nilead/v2.4.x

Fix linking error for Custom Comparison in document

maintenance release

Merge pull request #1724 from mcorteel/v2.4.x

Use mb_strlen to get string length to avoid unicode characters problems

PHP 7 String class is reserved

Merge pull request #1684 from spiroski/v2.4.x

Fix PHP7 error "Cannot use 'String' as a class name".

fix for materialized path node deletion

Merge pull request #1669 from freshcells/v2.4.x

Fix node deletion for similar paths with materialized path strategy (2.4.x)

fix for tree slug handler

Merge pull request #1650 from cmfcmf/unique-trees-2.4

[2.4] Fix unique slugs with the TreeSlugHandler

patch for nested tree sync method

Merge pull request #1647 from kbond/2.4-php7-nested-set-fix

PHP 7 nested set fix for 2.4

odm field annotation backported

v2.4.20

change odm string annotation to field with type string

regression for php 5.3 support

Merge pull request #1639 from Loscillo/Loscillo-patch-1

Fix incorrect syntax for arrays in PHP 5.3

recently introduced bug fix in mapping

Merge pull request #1635 from vbartusevicius/v2.4.x

fix for #1634