DATAMONGO-471 - Add support for $each when using $addToSet.

Additionally to Update.addToSet(String, Object) the method 'addToSet(String)' has been introduced, returning a builder to allow the creation of $addToSet command for either single value, or multiple values using $each.

Using value:
new Update().addToSet("key").value("spring");

Using each:
new Update().addToSet("key").each("spring", "data", "mongodb");

Original Pull Request: #157.