Merge pull request #70 from hectorj/master

correct typo in error message
added support for accessing array keys by other value objects, support for unsetting array members, support for array_key_exists(), isset() and unset() functions (feature request in from issue #71)
added unset function (request from issue #71)
added array_push function
Merge branch 'master' of https://github.com/CopernicaMarketingSoftware/PHP-CPP
fixed compile issues with gcc 4.7 (see issue #72)