Decouple identity map from object instantiation

[stve]

Modifications to Twitter::Base and Twitter::Identity to decouple the identity map from object instantiation which should fix #285

[sferik]

I'm happy to pull this now. The code seems like a net improvement and is worthy of being merged into master.

However, it leaves the issue of "child" objects (discussed here) unresolved. I'd like to figure this out before releasing version 3.2.0, but it doesn't necessarily need to be part of this pull request.

Do you want me to pull now or wait for that issue to be resolved?

[stve]

The implementation we've been discussing deviates quite a bit from the original intent of this pull request. It probably makes sense to merge in before it snowballs into something much larger. I'll make a new branch for the new changes.