DurationBuilder is not accessible

Class DurationBuilder is not accessible but it is useful in many occasions. I.e. I would like to create methods like

```
def f(db: DurationBuilder) = ... do something
```

and use it like f(1.second + 50.millis)

If I use it right now I get:
class DurationBuilder in package time cannot be accessed in package org.scala_tools.time

Can this class be made public?
