Don't use HashSet when deserializing images from mongo

Avoid using a HashSet when deserializing Images from mongo as the
ordering is inconsistent which means the readHash will be affected and
thus Content will be rewritten when it hasn't really changed. An
ImmutableSet.Builder is used instead.
Merge pull request #89 from atlasapi/image-persistence-order

Don't use HashSet when deserializing images from mongo