Assign workflow instance name/identifier

[Miggleness]

This PR is to address #153 which exposes the WorkflowInstance's 'Reference' property.

Notes:

• A number of unit tests are failing from my end prior to change, I didn't bother with those. I'm using Rider 2018.1 on Mac.
• Regarding placement of "Reference" argument in StartWorkflow(). The current placement feels awkward, but it sets up for slightly better compatibility. Preferably, the sequence will be (id, version, reference, data)
• I'm considering updating model builder to index 'Reference'. But then, probably better to let the end users decide on that.

[danielgerlag]

Thanks for the PR!
Could you also increment the version number?

[danielgerlag]

Could we simplify all the samples to just use host.StartWorkflow(name)?

[Miggleness]

done :)

Any chance you're upgrading the samples to dotnet core 2.1.1? For some odd reason, my Mac won't run the 1.x SDK installers.