Add ES `_parent` relation from items to their resources (see #246)

- Add `_parent` metadata to ES bulk format for JSON-LD import
- Tweak scripts (parent and children need to use the same index)
- Configure mappings for the `json-ld-lobid-item` ES type
Add `owner` query parameter to `resource` queries (see #246)

The `owner` specifies the ID of an organisation holding items of
the requested resources (eg. GET /resource?author=Hu&owner=DE-Sol1)
so the query will return only resources owned by the given org.
Tweak parent-child settings for resources and items (see #246)

- Move timestamp to process.sh (allow two jobs to use one index)
- Always add a parent to fix indexing of parent-less items
- Remove items context doc (resources context has all item props)
Update to ES 0.90.7, which changes parent/child handling (see #246)
Refactor search API to avoid excessive parameter list (see #246)
Add sample query for new `owner` parameter to main page (see #246)
Update Elasticsearch setup instructions (see #246)
Fix lodmill-ld integration test to include index type (see #246)
Merge pull request #283 from fsteeg/issue-246-api

Narrow down `/resource` queries to holding organisation