Configurable cell_class in DataTable

There are two changes in this proposal. One is to move the logic
for extracting cell data from the row datum (and handling auto columns)
to the Cell class, into a separate method for easy extending. The
second is to make that extending possible by making the cell class
configurable in the DataTable's meta.

Those changes are supposed to make it easier to extend the DataTable
with new kinds of cells -- for example, cells that contain an editable
widget, a progress bar, an icon, etc. as well as make it easier to
modify the behavior of the auto columns -- for example, by making it
possible to have some of the multi_select checkboxes preselected. Those
changes would also make it easier to integrate DataTable with a Django
FormSet.

Right now, in order to modify any of the proposed Cell.get_data
behavior, one has to make their own Row class and practically copy-paste
the whole Row.load_cells method, as there is no easy way to extend it.

Change-Id: I29277a9e77e1c413193fe80d3f8cfe001bf5d709
Closes-Bug: #1229677
Merge "Configurable cell_class in DataTable"