Metadata support

[martinklepsch]

Can be passed to putObject.

• rejected values in file-map can hold different types of values (File, map), validate those--
• find sane mapping from map to ObjectMetadata objects
• rejected diffing needs to be smarter, i.e. if no metadata is supplied assume unchanged
• input streams must be used for uploading

[martinklepsch]

Potentially a good tradeoff would be to not diff and only update if also the object has changed.

Diffing on metadata would require fetching metadata individually for each object which seems not particularly efficient.

[martinklepsch]

Custom metadata is now supported via a :metadata key in file-maps. If supplied metadata changes but the object (i.e. it's md5 checksum) doesn't the object and it's metadata will not get updated.