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

Additionally to 'addToSet(String, Object)' the method 'addToSet(String)' has been introduced, returning a 'Builder' allowing 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