Add support for computed fields

[yuvalek]

The code today does not support computed fields. When sqlalchemy-continuum tried to create the versioned row it also tried to write a value to the computed column thus making an error.

[yuvalek]

My pr excludes computed columns when writing but still creates them in the DB.

[yoshpe]

I've tested it, however the function versioned_column_properties isn't seemed to be called when generating the Metadata schema (the columns are still added to the history's table)

As a workaround, you can add 'exclude' to the __versioned__ key, and manually exclude the column, which works

[marksteward]

Yeah I'm not sure how we'd do this any way other than ignoring them. Suggestions welcome.

[yuvalek]

It is weird because I tested it locally when I wrote it and it did work and excluded the column.