Add scalding logo

Added Scalding logo to the README and also linked to the full artifacts:
https://drive.google.com/folderview?id=0B3i3pDi3yVgNbm9pMUdDcHFKVEk&usp=sharing

Signed-off-by: Chris Aniszczyk <zx@twitter.com>
Merge pull request #671 from twitter/add-logo

Add Scalding logo