Added a nuke task for copying public headers into a framework.
Added a nuke task for copying public headers into a framework.

Define a @public_headers ivar as a filelist of the .h files
and they will be copied into the framework.

timburks/nu,gcbeyond/nu,timburks/nu,spotify/nu,heiko-henrich/nutor,alloy/nu,spotify/nu,heiko-henrich/nutor,hodgeswt/nu,heiko-henrich/nutor,nulang/nu,nulang/nu,spotify/nu,hodgeswt/nu,alloy/nu,jqk6/nu,gcbeyond/nu,gcbeyond/nu,spotify/nu,hodgeswt/nu,jqk6/nu,jqk6/nu,timburks/nu,heiko-henrich/nutor,hodgeswt/nu,gcbeyond/nu,jqk6/nu,nulang/nu