Use type_to_sql in create_table & TIMESTAMP for dates

type_to_sql converts type names used by ActiveRecord to the ones used by
the database. For example, BigQuery uses TIMESTAMP to store time, so
":date" has to be converted to TIMESTAMP before the table is created
(this is another thing that this commit does).
Merge pull request #5 from vrok/master

Use type_to_sql in create_table & TIMESTAMP for dates