Hash buffer

Instead of reading the full file to compute the checksums, the read buffer is used to generate checksums with TransformBlock and TransformBlockFinal.

The performance ain't really change much but I would assume it has a larger impact on very large files.

![image](https://cloud.githubusercontent.com/assets/1127727/4359347/1ea3f04a-4271-11e4-8df6-2f5b7959c839.png)
These numbers are based on the test "ExecuteWithTimings" (Signature Create + Delta Create + Patch application) and I added another test 500 MB based on 5000 files.