Use ToStringHelper.add("field", field) instead of addValue(field)

Adding names to values improves readability of toString output.

From ToStringHelper.addValue JavaDoc:
Adds an unnamed value to the formatted output.
It is strongly encouraged to use add(String, Object) instead and give value a readable name.
Merge pull request #1 from mkiedys/add

Use ToStringHelper.add("field", field) instead of addValue(field)