Migrating mongeez from Log4j to SLF4J

Hi,

I am trying to use mongeez library in my project but I am forced to add the log4j as dependency although I am using the logback as my logging framework. Hence I propose to use the SLF4J as logging facade and leave choosing the logging framework to the users.

I have done a patch (Pull Request) for this update, kindly check it.

There is one downside when I migrated mongeez from log4j to SL4J, the log level can’t be set programmatically. In order to fix this issue for the unit tests (which set the log level to trace) I created a new log4j.properites in src/test/resources.