Report deleted & changed files at the end of the run

This should make seeing what the BFG has got up to a lot easier, and will make a dry-run mode (still not implemented) much more useful. See also https://github.com/rtyley/bfg-repo-cleaner/issues/26 &https://github.com/rtyley/bfg-repo-cleaner/issues/17#issuecomment-36127608

The new output looks like this (and it only appears if files *have* been changed, or deleted, as appropriate).

```
Changed files
-------------

	File                           Before     After
	--------------------------------------------------
	bushhidthefacts-ORIGINAL.txt | 93fd267a | 4f6f1558

Deleted files
-------------

	File        git-id     Size (bytes)
	-----------------------------------
	video.mp4 | 294f4016 | 126384
```