[FINAL] Validating hash when downloading/extracting bundles.

An attempt to validate bundle hashes as they come in the door.  I was able to sha1 hash the file contents as they stream in, avoiding secondary passes over the file contents.

This is a follow on attempt to #32
This should expect the specific exception class now.
Also, why not use the `@Test(expected = ValidatingDigestOutputStream.HashFailedException.class)` syntax instead?