Handle dependencies and main class with Maven Assembly plugin

I was having some trouble getting Shiro running because of dependency issues and an incomplete manifest in the jar. These changes add a plugin to handle creating a "standalone" jar that has all of the dependencies included and the main class set in the manifest. I'm not a Maven maven so any feedback on this is welcome. Based off of [this](http://stackoverflow.com/questions/1729054/including-dependencies-in-a-jar-with-maven#1729202).

This should allow:
    $ mvn package
    $ java -jar target/shiro-1.0-SNAPSHOT-standalone.jar ./example_code/example.sro