Make id field customizable

[toopy]

Then developper can define which field to use as index key in a convenient way.

[willkg]

This needs:

1. git commit messages that are formatted correctly (http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)
2. the commits should be squashed--this is really one single change
3. the commit should refer to issue The name of the primary key is hardcoded #265 which this fixes
4. this needs unit tests

I'll look at it after that's done.

[toopy]

up

[louiseGrandjonc]

Hello, is it possible to merge that PR because I'm using it and it's very usefull 👍

Bye