Adds method for retrieving all document attachments, changes Date serialization

*NOTE*: This PR changes the serialization format of `Date` objects to include milliseconds. The old format is added as a legacy format that `SerializationUtils` can deserialize, but not serialize to. This affects all configuration (stage, stagegroup) as well as document serialization.
Previously, sub-second components were lost in serialization, which made tests difficult to write and required weird workarounds in test code.

Legacy format: `2013-01-13T14:02:56+0100`
New format: `2013-01-13T14:02:56.123+0100`

**Adds `getFiles` to `RemotePipeline`, for fetching all attachments to a document**. This addresses #240, by making it easier to get the attachments.

I also introduced the use of WireMock for testing `RemotePipeline`, something that should be expanded to all methods in the class.