Adding a new error that will fire when trying to use One(), All() or Next() without closing the initial result set.

Better time.Duration to string conversion.

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)
```

Fixing db.Fields for MongoDB, related: https://groups.google.com/forum/?fromgroups=#!topic/gosexy/3QcW40qZYfk

sqlite: Benchmark applying BEGIN and END transaction optimizations