Async/future execution

[janm399]

Include Asynchronous execution policy that supports

val jdbc = new JDBC(ds) with Iteratees with Asynchronous
val pu = jdbc.select("<<USERS WITH ORDERS>>", list[User])(...)
val po = jdbc.select("<<THE ORDERS>>", list[Order])(...)

// do some work, while pu and po are executing

for (u <- pu) { 
  // pu now needs to be ready
}

po(0) // po now needs to be ready