Add method to duplicate existing target

Hi all.

It would be great to have `duplicate` method on `PBXNativeTarget`.

This would be handy when dealing with automated test, e.g. Calabash for iOS.
Test team could pick up latest app from the repo, locate main app target and duplicate it into many other targets, for Calabash, MonkeyTalk, RevealApp or any other test-related stuff the developers shouldn't care about.

For example `calabash-ios setup` does this job, but it' snot using `Xcodeproj`.

The way I intend to use it is to get the latest app code first, then duplicate main target into a bunch of test-specific targets, then update Podfile to add custom frameworks and pods to newly created targets. And all that with a couple of cli commands, so it can be used on CI server as well.