Rearchitecting project settings so header files are moved into common build folder during build process

In general, even for a static library, you don't want your customers accessing the header files directly from the source directory.

Instead, the build process in this PR selects which header files are "public" and copies them to [BUILD_DIRECTORY]/usr/local/include as part of the build process. This allows customers compiling the static lib source in their build process to access the files in the same way as someone who is using the straight build results, which is why with this change, you're able to use:

#import <GPUImage/GPUImage.h>

I also edited the example project to make use of this.

NOTE: Also remove the manual SYMROOT that was included in the GPUImage.xcodeproj. It's not generally considered good practice and in recent xcode releases, I believe they've actually removed easy ways of changing it.