Brainstem.Collection pagination

- Add `Brainstem.Collection#fetch` that follows `Backbone.Collection#fetch` signature and behavior as closely as possible while adding the ability to include Brainstem specific options
    - Adds `add` option in addition to Backbone's `reset` option for appending to the collection on fetch success
- Add new `Brainstem.Collection` pagination API
    - `getPage` for fetching a specific page
    - `getNextPage`, `getPreviousPage`, `getFirstPage`, and `getLastPage` which use `getPage` to traverse the collection  
- Update `Brainstem.Collection#loadMore` to use new pagination API internally, uses `add: true` for infinite pagination
    - Fixes :beetle: in `hasMorePages` callback argument (I believe this could be called "legacy" functionality)

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*Note: existing `loadObject`, `loadModel` and `loadCollection` functionality remains untouched to preserve backwards compatibility*.