Support `on_schema_change` feature on incremental model

[hiro-o918]

Problem

Currently, dbt-athena does not support on_schema_change option. closes #47

Solution

• Add custom column class AthenaColumn
  • Athena has different column type between DML and DDL (e.g. integer)
  • https://docs.aws.amazon.com/athena/latest/ug/data-types.html
  • To avoid error on alter table statement because of difference of column type
• Overwrite macro sync_column_schemas
  • Athena does not support drop columns statement, but replace columns
  • https://docs.aws.amazon.com/athena/latest/ug/alter-table-replace-columns.html
  • this causes inconsistency with original sync_column_schemas, so overwrite this macro on this adaptor