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.

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
-------------

	Filename                             Before & After
	--------------------------------------------------------------------------------------------------
	CODE.conf                          | e3aa4a56 ⇒ b9241055, 6fd90c18 ⇒ 1f390cd7, ...
	PROD.conf                          | 5a89032a ⇒ 38193000, 2611394d ⇒ 9c742f65, ...

Deleted files
-------------

	Filename                     Git id
	------------------------------------------------
	bg.jpg                     | d0ea4091 (2.0 MB)
	guardian_space002.png      | 24215b1e (1.2 MB)
```