Better formatted output for formatted sizes.

This tiny PR improves IMHO the rendering of `report_sizes(t)`:

Before:

```
2013-11-21 22:28:04,684 build/ol-whitespace.js: <<command>>
2013-11-21 22:28:09,757 build/ol-whitespace.js: uncompressed: 1859204 bytes
2013-11-21 22:28:09,897 build/ol-whitespace.js:   compressed: 299023 bytes
```

After:

```
2013-11-21 22:31:30,042 build/ol-whitespace.js: <<command>>
2013-11-21 22:31:34,611 build/ol-whitespace.js: uncompressed:  1859204 bytes
2013-11-21 22:31:34,611 build/ol-whitespace.js:   compressed:   299023 bytes, (saved 83.92%)
```