Publicly expose the data source property

[cdzombak]

(Issue moved from the README in 3a4cfae )

PhotosViewController's data source should be exposed publicly, so that clients can add/remove photos from it or provide a new data source.

This is a more general solution, and better follows Cocoa design conventions, than adding methods for adding/deleting photos on PhotosViewController itself as proposed in #139 and #140/#141.

To do this work properly, we must consider:

• Do we allow setting a new data source on PhotosViewController? If so, we must ensure the entire UI is updated appropriately. A lot of this setup is currently done during initialization, so this could be tricky.
• When photos are added, removed, or changed, how is the PhotosViewController notified? Again, we must ensure the UI is updated properly & left in a consistent state.

(As an alternative to mutable data sources, a more functional approach could make sense and reduce the need for a complex notification/delegate relationship coupling these two objects; you'd set the data source to a new, immutable data source to make any change.)

[cdzombak]

This relates to #214, too. I'm thinking through the design for this now since many feature requests and PRs would benefit from a clearer idea of data flow.