Upgrade slf4j to 1.6.1 from 1.5.8

Before landing my big changes from FLUME-283, I'd like to get an update in place to the current version of slf4j and correct an issue with how it is set up.

Right now, slf4j is shipping in Flume with slf4j-simple. That means that things using slf4j right now (Thrift and Avro generated code) won't really do much in the way of logging and that logging would be separate from what is currently being done with log4j.

I'm suggesting in this patch that we get rid of slf4j-simple and switch to slf4j-log4j12 so that things using slf4j log through log4j.

In Flume-283, I am moving everything from the log4j API over to slf4j and we can revisit at that point which logging API we want to actually be using.