Addressing #341 Redirecting logging to STDERR.

Logging to stdout may pollute output from `tsdb` resulting in inconvenient use, as pointed out by @dennismphil in https://github.com/OpenTSDB/opentsdb/issues/341. This change points the default logging destination at STDERR.

```
$ sudo -u opentsdb ./tsdb scan 2014/09/11-23:42:00 sum network.hosts --import > /tmp/import.txt
2014-09-13 00:55:13,329 INFO  [main] Config: Successfully loaded configuration file: /etc/opentsdb/opentsdb.conf
2014-09-13 00:55:13,505 INFO  [main] ZooKeeper: Client environment:zookeeper.version=3.3.6-1366786, built on 07/29/2012 06:22 GMT
2014-09-13 00:55:13,505 INFO  [main] ZooKeeper: Client environment:host.name=number5
2014-09-13 00:55:13,506 INFO  [main] ZooKeeper: Client environment:java.version=1.7.0_65
2014-09-13 00:55:13,506 INFO  [main] ZooKeeper: Client environment:java.vendor=Oracle Corporation
[...]
```
That way the result of the scan is ready for import, log messages don't have to be filtered out:
```
oozie@number5:/usr/share/opentsdb/bin$ head /tmp/import.txt 
network.hosts 1410498110 1 hostname=number5 host=number5
network.hosts 1410498375 1 hostname=number5 host=number5
network.hosts 1410498700 1 hostname=number5 host=number5
network.hosts 1410498973 1 hostname=number5 host=number5
network.hosts 1410499278 1 hostname=number5 host=number5
network.hosts 1410499581 1 hostname=number5 host=number5
network.hosts 1410499877 1 hostname=number5 host=number5
[...]
```