add limit to retrieve method in PipestatManager

[khoroshevskyi]

It would be nice to have limit method that allows user to specify limit of the rows that have to be selected from the database.
++
retrieve method has incorrect docstring:

[donaldcampbelljr]

Is this still actually an issue? This particular method is for Retrieving a result for a single record.

There are other functions that do allow for retrieving multiple rows select and get_records and they do have limit parameters.

[khoroshevskyi]

It's not super important, because we can use backend.select. But I would keep this question open

[donaldcampbelljr]

This is solved with PR #101