Show diff in error msg when repr is long.

When expecting collections from a web api, this will help locate small changes in behaviour.
Tested on 2.7 and 3.2 tox doesn't like my pip/virtualenv anymore. Hope Travis has more luck than me.

Also, could you bump version and push to pypi? I noticed the previous pull hasn't ended up in pypi yet.
Perhaps remove the space between the call to `isinstance` and the closing `)` on the conditional?