Add support for Firestore-specific data types

[pheekus]

Support for Timestamp, Bytes, LatLng and Path is currently missing. v1 must have at least basic support for these types using the following syntax:

{
  "$schema": "http://json-schema.org/draft-07/schema",
  "$id": "FirestoreTypes",
  "type": "object",
  "definitions": {
    "Timestamp": {},
    "Bytes": {},
    "LatLng": {},
    "Path": {}
  },
  "properties": {
    "date": { "$ref": "#/definitions/Timestamp" },
    "bytes": { "$ref": "#/definitions/Bytes" },
    "location": { "$ref": "#/definitions/LatLng" },
    "document": { "$ref": "#/definitions/Path" }
  }
}

Compiler could look for $ref in property declaration and add basic type checking based on the provided value. This way only refs defined as shown above would work, but that'd be enough until a better approach is found.

Docs would need to be updated for this as well.

[pheekus]

Closed by #6, merged into dev branch.