Adds method for getting all attachments for a document as DocumentFiles.

Changes serialization format for Date objects to include milliseconds. Non-millisecond date deserialization is still supported as a legacy format.
Adds REST-based tests for file-related methods in RemotePipeline.
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Merge pull request #276 from laserval/remotepipeline-getfiles

Adds method for retrieving all document attachments, changes Date serialization