DATAMONGO-1517 - Add support for Decimal128 BSON type.

Support Decimal128 as Mongo simple type if present. Decimal128 is stored as NumberDecimal.

class Person {

  String id;
  Decimal128 decimal128;

  Person(String id, Decimal128 decimal128) {
    this.id = id;
    this.decimal128 = decimal128;
  }
}

mongoTemplate.save(new Person("foo", new Decimal128(new BigDecimal("123.456"))));

is represented as:

{ "_id" : "foo", "decimal128" : NumberDecimal("123.456") }

src/main/asciidoc/reference/mapping.adoc

@@ -126,6 +126,10 @@ In addition to these types, Spring Data MongoDB provides a set of built-in conve
 | native
 | `{"value" : { … }}`
 
+| `Decimal128`
+| native
+| `{"value" : NumberDecimal(…)}`
+
 | `AtomicInteger` +
 calling `get()` before the actual conversion
 | converter +