use sl4j logging api instead of log4j logger directly

[GoogleCodeExporter]

hi, it's common sence to use the sl4j logging facility in todays 3rd party 
jars. it would be good, if u can change the dependencis of the javapns lib to 
that only the api is used (compile dependency).

         <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>[1.6)</version>
            <scope>compile</scope>
         </dependency>

and the user of your lib can then use one of the different slf4j logging 
implementations by simple adding log4j, logback or whatever as a runtime 
depency like 

         <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>x.x.x</version>
            <scope>runtime</scope>
         </dependency>

of course, i could build javapns from src and make the changes for me, but i 
think woul be a nice enhancement.

- we use the version 2.2
- Red Hat Enterprise Linux Server release 6.5 (Santiago)

Original issue reported on code.google.com by marko.la...@googlemail.com on 30 Jul 2014 at 8:34