Generate default root value when creating new files #136
Description
As described in the top-level #98, once write support is working for at least the asdf/core datatypes the default behavior then for a new file should be a root value of type core/asdf, a history entry, and a software entry similar to the default behavior when creating a new file with Python asdf.
We should also add a convenience function for adding history entries.
I wonder if we should do anything with ManfestExtension... To be honest I still don't know exactly what that's for. I've looked into it a few times but off the top of my head I'm still confused about it.