Adds ´validationQuery´ configuration parameter

This adds a new `validationQuery` configuration parameter. If the `jdbc-connection` configuration doesn't have a `validationQuery` field then it is set to `null` (`ConnectionPoolSettings` would still work as expected [according to its default value](https://github.com/scalikejdbc/scalikejdbc/blob/develop/scalikejdbc-core/src/main/scala/scalikejdbc/ConnectionPoolSettings.scala#L25)).

This may solve issue #9 according to [a suggestion  by Victor Klang](https://groups.google.com/d/msg/akka-user/xRappYKjoho/3sYnCk0X9K8J). We are having this issue when using Oracle, so (for us) the solution maybe is to have a `validationQuery`config field set to `select 1 from dual`. I'm not sure if this is the solution, I'm only following Victor's suggestion. Also, I'm not sure if this same problem occurs with other DBs.