Fix up and improve RB.ResourceCollection.

RB.ResourceCollection was documenting and supporting the old-style way
of opting in/out of resetting the collection during fetch. Backbone.js
changed how this worked. We now support the modern way, and document it.

The modern way (Backbone.js 1.1+) is to use 'reset' instead of 'remove'.
Resetting is also opt-in in Backbone.js 1.1, though we flip that for
this collection to make it opt-out. We're not currently using
Backbone.js 1.1, but we want to stay close to this semantics to smooth
over the upgrade process.

It's also pretty useful to be able to specify the maximum number of
items to fetch in each batch. Callers could do this by passing
extraQueryData['max-results'], but that was less than ideal. There's now
a maxResults option that can be passed instead.

There's also fixes for the pagination indicators, hasNext and hasPrev,
which could appear as 'undefined' depending on the payload.