Added CheckboxColumn for datagrids

This commit adds a new CheckboxColumn class inheriting from Column.
When added to a DataGrid this column will contain checkboxes, with a
checkbox in the column header that can check/uncheck all the checkboxes.

The is_selectable & is_selected functions can be overridden to control
whether a checkbox is displayed in a row and whether that checkbox is
initially checked.

The checkboxes have a data-object-id attribute that should allow
javascript code to determine which rows have been checked and use that
accordingly.

This should allow mass submit/discard buttons to be easily added to the
dashboard in reviewboard.

- Integrated into revivewboard (code in this review: http://reviews.reviewboard.org/r/3799/ )
- Confirmed that header checkbox selects/deselects properly
- Confirmed that header behaves as expected (movable, displays correctly etc.)
- Confirmed that it's possible to write javascript to check which rows are selected and act on it.