[wip]Adding Exif node.

Credits are also @dceejay's as he started the initial work. This pull request resolves #78.

@knolleary, marked as [wip] as the error paths have not yet been tested manually. Unit tests are to be added.
This is @dceejay's colour choice. @knolleary, do you think this should be a bit paler green? I'm thinking of somethign like #a6f6a6 .