fix bugs https://github.com/go-xorm/cmd/issues/3

Signed-off-by: 商讯在线 <swhbox@foxmail.com>
improved Sync2 method
Merge branch 'master' of github.com:go-xorm/xorm
bug fixed for postgres dialect
bug fixed for sqlite3 dialect
add TODO comments
Merge branch 'master' of github.com:go-xorm/xorm
Merge branch 'master' into feature/cacher-tag