Add iterators in addition to `GitDiff::Diff#files` and `GitDiff::File#hunks`

[dadooda]

Hi Andrew,

At the moment methods like GitDiff::Diff#files and GitDiff::File#hunks return pre-filled Arrays, which isn't that optimal if you deal with big input data. I think adding iterators like #each_file, #each_hunk etc. would make it a lot more optimal performance-wise and memory-wise.

Cheers.

Alex

[anolson]

Hey Alex,

This seems like a pretty reasonable idea. I should be able to get something together fairly soon.