Relationships - improvements

[dread]

Suggested by Richard Cyganiak in ​http://lists.okfn.org/pipermail/ckan-discuss/2012-January/001977.html

• new relationship type links_to / linked_from
  • Maybe with extra 'count' property?
• creating a new relationship should be post to the relationship register, not the source entity.
• add missing doc about creating a new relationship
• 'Model Formats' section seems to be missed out of the docs since release 1.4.2. See: ​http://readthedocs.org/docs/ckan/en/ckan-1.4.2/api/version2.html#model-formats

[dread]

I propose spending 2h on this.

[dread]

• links_to/linked_from was already available (since Sept - Friedrich put them in).
• Count property not easy so not done.
• POST to the Relationships Register (all three) works now (and the original way via the entity still works too)
• Schema/validation was added along with tests for error conditions.
• Creating a new relationship documented in Model Methods
• 'Model Formats' section in API docs restored and updated.

Committed to master aimed for release 1.6 ​https://github.com/okfn/ckan/commit/f16450c1c34cac09046a098273f66d2b75aa87dd

Spent 6h on this in the end - generally useful tidying in this area.

[dread]

Went into CKAN release 1.6