Should WRAPPER_NAME replace PRODUCT_NAME during PackageApplication?

In [build.rb](https://github.com/mattt/shenzhen/blob/master/lib/shenzhen/commands/build.rb), `PRODUCT_NAME` is being used where I think `WRAPPER_NAME` would be more appropriate. By default, `WRAPPER_NAME` is `PRODUCT_NAME`+`WRAPPER_SUFFIX` (e.g. FooBar.app)

For some of my projects, I override `WRAPPER_NAME` to change the name of the final product file created by Xcode. This is based on [Xcode Build Settings Reference](https://developer.apple.com/library/ios/#documentation/DeveloperTools/Reference/XcodeBuildSettingRef/1-Build_Setting_Reference/build_setting_ref.html#//apple_ref/doc/uid/TP40003931-CH3-SW105) stating:

> PRODUCT_NAME
> Description: Identifier. Specifies the name of the product the target builds.
> 
> WRAPPER_NAME
> Description: Filename. Specifies the filename (including the appropriate extension) of the product bundle.

This works fine when I build everything through Xcode, but it fails when I attempt to build with shenzhen with the following error:
```
          xcrun  PackageApplication
error: Specified application doesn't exist or isn't a bundle directory : …
```