Adding cleaner method to remove worksheet

This takes account of removing defined names that refer to the deleted sheet, but doesn't consider any other formulae etc

lib/rubyXL/convenience_methods.rb

@@ -26,6 +26,18 @@ def add_worksheet(name = nil)
       worksheets << new_worksheet
       new_worksheet
     end
+
+    # Remove the worksheet with the supplied name
+    #
+    # @param [String] The name for the worksheet to be deleted
+    #
+    # There may be formula/references/links to the deleted sheet, which will be reported when opening the resulting file
+    def remove_worksheet(name = nil)
+      unless name.nil?
+        worksheets.delete_if{|ws| ws.sheet_name == name}
+        defined_names.delete_if{|dn| dn.reference =~ Regexp.new("^#{name}!")}
+      end
+    end
 
     def each
       worksheets.each{ |i| yield i }