Fixes #248 - phonegap-2.4.0 terminates in IOS

With 2.4.0, NSJSONSerialization is used, thus objects returned are immutable: http://developer.apple.com/library/ios/#documentation/Foundation/Reference/NSJSONSerialization_Class/Reference/Reference.html

Any NSDictionaries from the arguments should be considered immutable.
Release the mutableCopy of the NSMutableDictionary for non-ARC
Merge pull request #251 from shazron/cordova-2.4.0

Fixes #248 - phonegap-2.4.0 terminates in IOS