fix time.Duration convert to string. Reserved to the millisecond
In all the mysql and sqlite and postgresql wrapper,fun toInternal in collection.go.

Reserved to the millisecond:
```
return fmt.Sprintf(TimeFormat, int(t/time.Hour), int(t/time.Minute%60), int(t/time.Second%60), t%time.Second/time.Millisecond)
```
Merge pull request #26 from achun/master

Better time.Duration to string conversion.