parse out multiple references into separate ids

[sbenthall]

When there are multiple References in an email, these should be parsed out into a list of message IDs.

When these dictionary representations are turned into graphs, these multiple References should be processed as multiple edges rather than aggregate into a single output node.

[sbenthall]

the utility function exists for this in the bigbang.parse module. But since the data structure for list representation hasn't been finalized yet, this is still in limbo.