New column resizing behavior and modes.

Splits resizing behavior into two modes (standard and fluid), configurable via:

columnMode: 'standard/fluid'

In standard mode, columns push and pull their neighbors; resizing a column will
change only that column's width.

In fluid mode, columns steal width from their neighbors when resized.

Column configuration options have been made more effective and consistent:
minWidth/maxWidth now work reliably, and isResizable/isSortable/canAutoResize
can be configured to get a wide range of behaviors out of Ember Table. Width
can be persisted via the savedWidth property.

Finally, adds a "configurable column demo" that makes it easy to play with the
new configurations to see what they do or find a behavior that will work for
different use cases.