Restructure with OOP in 7.x-2.x

I've been meaning to re-structure some parts of the module to better-leverage OOP, and ensure things are a bit more flexible for ongoing development. Furthermore, now I've started to explore better file support and Media module compatibility (#2055737: Expanded support for Media module - Allow file fields to be used for titles and captions and #2087989: Views plugin support for views based on files) I see that several current issues would be a lot easier to address with better structured classes and methods in place.

In the interest of time I was going to hold off until the D8 port to any kind of re-architecture work, but now I'm thinking it'll be best to get a 7.x-2.x branch in place, that uses OOP as much as possible, ASAP. This might also get things closer to a structure that's more friendly to a D8 port, so exploring 7.x-2.x in parallel to framing up 8.x-2.x might make good sense.