Relationships: Extend with custom data AND create Note API AND provide UI for entering/editing a Relationship note

1. Implement custom groups/custom fields/custom data for Relationships
We should be able to clone the work we did to support custom fields and data for Activities. If a custom_group extends Relationship, it's fields appear in a fieldset on the Add/Edit Relationship form/view below the new Note field (see # 3 below).

2. Note APIs
Implement APIs for Note class. In general, a Note can be attached to any class/object (via entity_table/entity_id) - so we'll need to support that. We need the following functions:

• crm_create_note($params)

• $params is an array with entity_table, entity_id, note, and contact_id values required. modified_date defaults to current date but can be passed in $params.
• returns a single array with the created note (ID and properties)

• crm_get_note($params)

• $params is an array with EITHER the ID of a single note to retrieve: ( 'id' => $noteID) OR to ask for all notes for a specific entity (e.g a contact or a relationship or ...) - array has ( 'entity_table' => $table, 'entity_id' => $id). A specific note ID could also be passed in this second case but then the other 2 params are redundant.
• returns a nested array of 0 or more Note(s) / Note properties.

• crm_delete_note($params)

• $params is an array with the ID of the note to delete: ( 'id' => $noteID) OR it can consist of the entity_id/entity_table combination (both required) which would delete ALL notes for the specified entity.
• returns the number of records deleted OR an error array with 2 elements:
  ('status' => 'Error', 'message' => $errorMsg)

• crm_update_note($params)

• $params is an array with ID of a note to update: ( 'id' => $noteID) AND the property => value pairs "to be updated"
• returns a single array with all the note properties (updated)

3. UI support for adding a Note (civicrm_note) instance when adding or editing a Relationship. (For now we'll just support one note per relationship in the UI.)

• Add a Note field (textarea) to the Add/Edit/View Relationship Form (below End Date).
• When adding a Relationship - if the Note field is populated, create a new note record linked to the new relationship.
• When editing a Relationship - query for a Note for this relationship (entity_table = civicrm_relationship, entity_id = $thisID). Order the query by modified date DESC - so if more than one note exists for the relationship (which could happen via API inserts) - we'll grab the most recent.
• Same for viewing a Relationship