Generate MD5 hash of contents to verify uploads.

S3 will verify the uploaded contents when the Content-MD5 header
is given. For multipart upload, a hash of each uploaded chunk
is given, and the whole upload fails if any chunk fails.
Fix default value for chunk-size
Bump to version 0.0.3
Merge pull request #2 from mikepilat/master

Verify upload integrity, plus a bugfix