Metadata spec and JSON schema

This is a minor update release to the V4 metadata spec. Changes in V4.3.0 include:

1. Increased max state_of_specimen.ischemic_time and state_of_specimen.postmortem_interval value to 1000000 (#89)
2. Added "10X" and "10x" as options to seq.library_construction, rna.library_construction, and single_cell.cell_handling
3. Updated sample.biosd_sample regex to ^SAM(D|N|E([AG]?))[0-9]+$ (#78)
4. Added "Cambridgeshire" to list of accepted enum values for contact.country_division
5. Updated all URLs to reference 4.3.0

All changes maintain backwards compatibility.

The V4 metadata schema is found here:
https://docs.google.com/document/d/12BTAyZip9Q3r0Nm-L3ws6WzMmVi_kmUUcvOTrO47l2s/edit

The V4 metadata schema was developed by the HCA metadata team at EMBL-EBI. It consists primarily of structural changes to the V3 schema to make the HCA metadata schema fully compatible with JSON schema and facilitate automatic schema validation. The release contains some sample data for testing and the metadata team are currently updating previous example data from the Q3 demo to conform to the V4 metadata schema.

A template spreadsheet to capture V4 metadata can be found here: https://docs.google.com/spreadsheets/d/1OboVETG6lQpdRm2-m5uRbk1gk80o_Yo2FqSzXDb_VxY

A document detailing the field-by-field changes from V3 to V4 can be found here:
https://docs.google.com/document/d/1DouNpWH6xaFICqPZ6fwxzfPzJOmf5FUuQ8h_MrDEKdA

A high-level overview document of V3-to-V4 changes can be found here:
https://docs.google.com/a/ebi.ac.uk/document/d/12BTAyZip9Q3r0Nm-L3ws6WzMmVi_kmUUcvOTrO47l2s