Missing requirements in JSON Schema

[HadrienGardeur]

The following requirements in OPDS 2.0 are currently missing from our JSON Schema:

• [x] A feed must contain a reference to itself using a self link in links
• [x] Each Link Object must contain a title (navigation)
• [x] Each facet collection must contain a title in its metadata
• [x] Each group must contain a title in its metadata
• [ ] Validate the presence of price for specific acquisition links

[HadrienGardeur]

Last two were already covered indirectly.