[SPARK-2952] Enable logging actor messages at DEBUG level

Example messages:
```
14/08/09 21:37:01 DEBUG BlockManagerMasterActor: [actor] received message RegisterBlockManager(BlockManagerId(0, rxin-mbp, 58092, 0),278302556,Actor[akka.tcp://spark@rxin-mbp:58088/user/BlockManagerActor1#-63596539]) from Actor[akka.tcp://spark@rxin-mbp:58088/temp/$c]

14/08/09 21:37:01 DEBUG BlockManagerMasterActor: [actor] handled message (0.279 ms) RegisterBlockManager(BlockManagerId(0, rxin-mbp, 58092, 0),278302556,Actor[akka.tcp://spark@rxin-mbp:58088/user/BlockManagerActor1#-63596539]) from Actor[akka.tcp://spark@rxin-mbp:58088/temp/$c]
```

cc @mengxr @tdas @pwendell

Author: Reynold Xin <rxin@apache.org>

Closes #1870 from rxin/actorLogging and squashes the following commits:

c531ee5 [Reynold Xin] Added license header for ActorLogReceive.
f6b1ebe [Reynold Xin] [SPARK-2952] Enable logging actor messages at DEBUG level