Header automatically select when all rows selected

When the user selects all the rows of the grid, the header cell will
automatically update its state to selected. This keeps it consistent
with the state of the grid. This is accomplished by comparing the length
of the collection with the number of selected models.
Add unit tests
Fixed IE 8 compatibility issue
Remove excessive events in tests
Merge pull request #7 from pmaidens/auto-select-all

Header automatically select when all rows selected