Only include *.xcassets from Pods

Fixes #1546

If you had multiple xcassets anywhere underneath your root project directory, but were only picking one of them for each build, the `Pods-resources.sh` script would include ALL of them in the build. This could cause resources from the correct xcassets to be overwritten by all the others.

Now, `Pods-resources.sh` will only include the xcassets found in `Pods`. All other xcassets can be included in the build by adding them to the Xcode project, as per usual.