Call new AkamaiPurge when user does not

Instead of throw an error in AkamaiPurge when not called as a constructor, should instead just recursively call itself with new. This prevents the user from unnecessarily having to call it with `new akamai.purge...`, an easy mistake to make.
Updated test.
Merge pull request #5 from matthewp/call-constructor

Call new AkamaiPurge when user does not