[Initial Review] Default Mode: New column resizing, part 1

For initial review. Please ignore the new resizable example; it'll be tweaked and polished before merging into master. Focus on changes in the src folder, which defines the actual ember-table behavior.

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

@bigsley @ldevalliere