Implement toSingleBox.

The toSingleBox method takes a List[Box[T]] and turns it into a
Box[List[T]], via an implicit conversion to ListOfBoxes.
Simplify the toSingleBox implementation.
Update toSingleBox doc to state that List sizes may differ.
Merge pull request #1525 from lift/msf_issue_1525

Implement a helper for Iterator[Box[T]] => Box[List[T]]