Add a high level API for editing checklist items.

Hi,

For an integration I'm working on, I found the need to edit checklist item names and their state, but didn't find any nice way to do that via this wrapper. The API does have (slightly odd) methods for making this change, so I tried to wrap them up nicely.

## Commit message

Until now the only way to mark a checklist item as complete
was to remove it and re-add it which is certainly sub-optimal
and also messes with the ordering of items.

The exposed API does have a mechanism for updating the name
and state of checklist items but it's non-obvious and
requires that the both the card id and checklist id
are known.

This commit adds a highlevel API to do this renaming and
state changing.

In order to not break API, a new method called 'getItemObjects'
has been added to supplement the 'getItems' method of the
Checklist class. This new method will return objects rather
than a plain dict and exposes 'updateName' and 'updateState'
methods.