Feature/28/multi resource lock (#55)

* Add support for locking multiple resources

Locking of multiple resources in single call is "hard" due to nature of redis. Even when using MULTI (transaction support) all commands which are queued successfully will be executed (so we can't rely on redis to fail on already set key).

This commit provides a sort-of-workaround by checking existence of values but it still doesn't provide strong guarantees for settings/removing/extending all resources.

Ref #28

* Change idention to tabs

* And rework some more tabs

* Tab Tab

* Set nil on lock error and check value diff on del

* Add multivalue tests