Feat: Stored Procedures

(I saw a comment on the README along the lines of supporting Callable statements. Assuming this is where you were heading with that?)

Maybe something like:

``` java
String state = db.
.call("{call GetStateByZip(?)}")
.parameter(90210)
.getAs(String.class)
.first().toBlocking().single();
System.out.println(state); // California
```

Stored procedures would support named parameters, as already supported in other queries.