Add ability to delete session forcibly

I now need to delete session by doing like this.

``` go
session.Options(sessions.Options{MaxAge: -1})
session.Clear() // or whatever changes session.written to true
session.Save()
```

I think it would be better if
- DeleteSession() method is added<br>
  or
- Save() method takes a parameter to save session regardless of the session.written flag

I'm sorry if I'm missing something.
