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.
Avoid object creation with:

```
result.offer(Hashing.sha256().newHasher()
   .putBytes(q.poll().asBytes())
   .putBytes(q.poll().asBytes())
   .hash());