Support for const property validation

Today all `const` fields in the schema are ignored. This information should instead of be used to validate properties have the expected values:

Example schema:
```
{
  "type": "object",
  "properties": {
    "name": {
      "type": "string",
      "const": "foo"
    },
    "value": {
      "type": "number",
      "const": 42
    }
  }
}
```