Add norman field attribute "pointer"

Add a new field attribute "pointer" to indicate that the generated
client code for the field must be a pointer. This allows clients to
differentiate between sending nil/leaving the value unset and sending an
empty map or slice.

This change also removes the `nullablestring` norman type introduced in
30f8d18 since schemas that need a pointer to a string can now use this
field attribute. There are no libraries currently using this feature so
it should be safe to remove.

Example usage:

```
Labels map[string]string `json:"labels" norman:"pointer"`
```

Resulting API schema:

```
"labels": {
  "create": true,
  "nullable": true,
  "pointer": true,
  "type": "map[string]",
  "update": true
}
```

Generated client code:

```
Labels *map[string]string `json:"labels,omitempty" yaml:"labels,omitempty"`
```

parse/builder/builder.go

@@ -382,8 +382,6 @@ func ConvertSimple(fieldType string, value interface{}, op Operation) (interface
 		return convert.ToString(value), nil
 	case "reference":
 		return convert.ToString(value), nil
-	case "nullablestring":
-		return convert.ToString(value), nil
 	}
 
 	return nil, ErrComplexType