Fix incompat with Backbone master: add `|| options.remove` to `add` overload.

Also, fix non-working short circuiting for non-RelationalModels on collection methods.
Rename findOrCreate `update` option to `merge`, like Backbone's naming.

Backbone uses `update` on fetch to indicate the update method should be called,
and `merge` on add to indicate models should be updated.
Update test for the change from findOrCreate `update` to `merge`
Backbone master compat: guard against models that weren't actually added
Small performance improvement on `remove`; call once, instead of in a loop.
Document `findOrCreate.merge`, add a change log, fix some display issues.
Fix `add`, `merge` and `remove` options on coll.add or relation.set. Ref #224
Add docs for `RelationalModel.set`
Only modify `this.related` through `setRelated`; restore an accidentally commented-out test.
Docs: use more logical link names for attributes of `RelationalModel.relations`
Couple of small simplifications
Upgrade underscore to 1.4.4; replace `detect` with `findWhere`.
Collection._prepareModel: only `_validate` new models