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