Make schema store optional

[jdeolive]

Short term fix would be to add a memory based implementation.

[newmanw]

Either make this optional or provide documentation on the format of this mystery file.

[tkunicki]

@jdeolive does boundless have the documentation i'd created for this extension? The file JSON/BSON object used to represent the schema was described in detail.

[karl-potisepp]

I would be grateful for any documentation with regard to the schema file format.

Edit for future generations: the schema store parameter expects a folder (or I suppose a Mongo collection), where GeoServer can store it's schema information. No mystery schema files are needed from the user.