s3: Add support for using Content-MD5 with PutReader.

RFC 1864 specifies that this header field is base64 encoded.
This is slightly odd since ETag is hex encoded.
Merge remote-tracking branch 'upstream/master' into pr/content_md5

Conflicts:
	s3/s3.go
Fixing merge breakage
Revert change, unit tests pass
Merge pull request #116 from spiralgenetics/pr/content_md5

s3: Add support for using Content-MD5 with PutReader.