Additional node properties for standardization

[dkoslicki]

The specification notes these properties:

1. category [required]: A semantic type from the biolink model
2. description [optional]: Narrative text

I'm strongly in favor of including the description property as well.

@saramsey and @DeqingQu, will the node properties you're working on pulling include these?

NOTE BY SAR: Item (2) ("description") is now a separate issue, #81

[saramsey]

working on this now

[saramsey]

I am breaking out item (2) above to a separate issue (#81) since it is nontrivial; Item (1) is easy and I am doing it now.