Serialization Part II

[jlowin]

Continuing #310:

• Uses new Schemas for Edge.serialize(), Task.serialize() and Flow.serialize()
• Splits Schemas into separate files for clarity (core.py -> {flow.py, edge.py, task.py}) -- this will be mostly beneficial when there are multiple versions of schemas in the same file
• Adds a new schema just for Parameters
• Parameter classes now accept description and tags because why not?

NOTE: Cloud is not yet updated to ingest the output of Flow.serialize(). It should be a minor change since the payload is almost identical, but master/master integration will break with this PR until that is changed.

[cicdw]

I like how this is looking