Allow for different numbers of items per line depending on collection view size.

[klundberg]

If the scroll direction is vertical, varying widths should allow for different items per line, something like this:

• width 0 -> 200: 2 per line
• width 200 -> 500: 3 per line
• width >= 500 -> 4 per line
• etc

The same applies for horizontal, except the height would be taken into account.

[klundberg]

This may be extraneous, since as of 0.4 the delegate methods can be implemented to handle this.