support for storing & retrieving custom metadata

This commit adds support and a test for storing and retrieving
custom metadata for S3 objects, as documented at:

    http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPUT.html
Amazon authentication: Fix Expect: 100-continue

- Respond with 100 Ok if the client has "expect: 100-continue" in header
Merge remote-tracking branch 'refs/remotes/hblanks/master'

Conflicts:
	lib/fakes3/file_store.rb