[Sandbox::FileAccessor] Add support for private_header_files

Closes #998

lib/cocoapods/sandbox/file_accessor.rb

@@ -80,11 +80,13 @@ module Pod
       #
       def public_headers
         public_headers = paths_for_attribute(:public_header_files)
+        private_headers = paths_for_attribute(:private_header_files)
         if public_headers.nil? || public_headers.empty?
-          headers
+          header_files = headers
         else
-          public_headers
+          header_files = public_headers
         end
+        header_files - private_headers
       end
 
       # @return [Hash{ Symbol => Array<Pathname> }] the resources of the

spec/unit/sandbox/file_accessor_spec.rb

@@ -73,6 +73,14 @@ module Pod
         ]
       end
 
+      it "filters the private headers form the public headers" do
+        @spec_consumer.stubs(:public_header_files).returns([])
+        @spec_consumer.stubs(:private_header_files).returns(['**/*Private*'])
+        @accessor.public_headers.sort.should == [
+          @root + "Classes/Banana.h",
+        ]
+      end
+
       it "returns the resources" do
         @accessor.resources.sort.should == [
           @root + "Resources/logo-sidebar.png",