Revert "Merge pull request #2 from tan-c/database_migration"

This reverts commit 8d8ecb7c12666192a7f1923cd930741a2c69403e, reversing
changes made to f64d9f6f5b458e76fa9d0ced7585afeb06ca818b.
Revert "refactor the database on 20140124"

This reverts commit f64d9f6f5b458e76fa9d0ced7585afeb06ca818b.