Add direct support for edit cell with objects e.g. Timestamp

[richardlin047]

Right now if you edit a date column, the history will look like:

# Edit cell
df.loc[(1, 'Date')]=Timestamp('2013-01-15 00:00:00')
unfiltered_df.loc[(1, 'Date')]=Timestamp('2013-01-15 00:00:00')

If you call spreadsheet.apply_history(), Timestamp is not defined, so it doesn't work. Would need to change history to include the import name or import objects to be supported.