Add fluent addColumns operation to CsvSchema.Builder

[ansell]

The current CsvSchema.Builder doesn't support the fluent addition of multiple fields in a single operation. This makes it necessary to break out of the fluent interface temporarily to loop through field names before completing the build.

This could be supported through either 1 or 2 new operations added to CsvSchema.Builder:

        public Builder addColumns(Iterable<Column> cs) {
            for (Column c : cs) {
                _columns.add(c);
            }
            return this;
        }
        public Builder addColumns(Iterable<String> names, ColumnType type) {
            Builder result = this;
            for (String name : names) {
                result = addColumn(name, type);
            }
            return result;
        }

Note that CsvSchema itself would match Iterable<Column>, which may or may not be what is most intuitive as it wouldn't copy across all of the other values as the Builder(CsvSchema src) constructor does.

[cowtowncoder]

Fixed via #131 and as per comments, officially included in 2.9, but technically will be included in 2.8.2 already (just not documented). This because master hasn't yet moved to 2.9.0-SNAPSHOT (will do so within next week or so).