schema: Add Droonga::Catalog::Schema
schema: Add Droonga::Catalog::Schema::{Table,Column}
schema: Fix typo on class name

Columns ->
Column
      ^
schema: Add readers for table options
schema: Check equality with inner data values
schema: Add ColumnIndexOptions class
schema: Add readers for Column
schema: Fix place of sources option
schema: Implement Schema#to_commands
schema: Add a test for Table#flags
schema: Remove "with" from column index options
schema: Add tests for reader of Column
schema: Add tests for reader of ColumnIndexOptions
schema: Insert type_flag method
schema: Add Column#flags
schema Add test for ColumnIndexOptions#position
schema: Make flags methods to return an array
schema: merge flags from ColumnIndexOptions
schema: Fix indexOptions attribute name
schema: Add tests of flags for columns