JCLOUDS-457: Added TreeHasher util class and its test.

The TreeHasher class allows us to calculate the linear hash and the
treehash of the archives and multipart upload parts.
You can write this more succinctly as:

```
List<HashCode> list = Lists.newArrayList(ImmutableSortedMap.copyOf(map).values());
```

You might not need the `ArrayList` copy if you change some of the methods to take a `Collection` instead.