Define a bco specific uri object

[HadleyKing]

schema is valid according to object and @corburn's validate.py script.

Fixes #5

Define a bco specific uri object and update the MD files.

this fixes #23 and fixes #48 -> JSON schema validated the uri and access_time.

digital_signature -> checksum

fixes #43

removed "Specification" from MD files and renamed as User Guide.

• Updated the README to reflect User Guide

edits for consistency between schemas and User Guide.

fixes #42 and fixes #50