[SUGGESTION] Make addField public (AddField)

[JamMika]

It is very useful to have AddField public for the case when you want to convert encoding like e.g.:

	for i := 0; i < len(inTbl.Fields()); i++ {
		fieldName := inTbl.Fields()[i].Name()
		fieldType := inTbl.Fields()[i].FieldType()
		length := inTbl.Fields()[i].Length()
		decimalPlaces := inTbl.Fields()[i].DecimalCount()
		outTbl.AddField(fieldName, fieldType, length, decimalPlaces)
	}

dbftable.go line: 149

func (dt *DbfTable) AddField(fieldName string, fieldType DbaseDataType, length byte, decimalPlaces uint8) (err error)