add source_map getter

Author: Tyler Horth

lib/sassc/engine.rb

@@ -49,6 +49,7 @@ def render
       css = Native.context_get_output_string(context)
 
       @dependencies = Native.context_get_included_files(context)
+      @source_map   = Native.context_get_source_map_string(context)
 
       Native.delete_data_context(data_context)
 
@@ -62,6 +63,11 @@ def dependencies
       Dependency.from_filenames(@dependencies)
     end
 
+    def source_map
+      raise NotRenderedError unless @source_map
+      @source_map
+    end
+
     def filename
       @options[:filename]
     end