Fix multiselect bugs

Make selection contain the contents of rows, rather than the row objects
themselves, and make a new Ember.Set for each EmberTableComponent

Closes #192
Default Mode: New column resizing, part 1

This is a work in progress and will not be merged into master without
discussion of the new spec.

* Avoid reszing columns to widths less than 0, which breaks the table
* New behavior uses canAutoResize when forceFill is off.
* Use iterative resizing to compute column widths in case of min/max
  width issues
* Removed 500px limit on max width during resize