add the option to package the AOT-compiled classes into a jar

#121:  package classes into a a jar instead of bundling them as classes in WEB-INF/classes.  
 
This is important for those with fairly large projects (or projects that pull in a lot of dependencies that get AOT-compiled).
This can actually lead to too many open files depending on the file descriptor limits of your OS.  Yes, you can bump the FD limits at the OS level, but this is much friendlier.
Normal behavior is unchanged, but if you set `{ :ring { :jar-classes true }}` in your project.clj, it will invoke the lein jar task and then include that jar in WEB-INF/lib instead of putting the bare classes in WEB-INF/classes.  Runtime behavior should be unchanged, although this does put your classes in a different order on the classpath, so we can't rule out odd behavior if you have classpath conflicts.