Add 'time_format' to return formatted time value

Add the format of the time written in database.

I wrote this pull-request to store `time` such DATETIME or TIMESTAMP format into a database. 

By this fix, dbi-plugin will treat `time` key  as formatted-string or unix time value(if time_format is not specified)
 
usage:
```
<match dbi.*>
  type dbi
  dsn DBI:Mysql:dbname:dbhost
  db_user username
  db_pass password
  keys host,time,method,uri,protocol,status
  query insert into access_log (host, time, method, uri, protocol, status) values (?, ?, ?, ?, ?, ?)
  time_format %Y-%m-%d %H-%M-%S  
</match>
```

How do you like?