Architecture changes

-Changed plugin importer to be static, so that a loader can be defined
at every call
-This may not be necessary, but for now it will help with the multiple
calls
- The non static methods will be removed
- Fixed a typo in CreateJarFile class
- Added calls to examine jar files in every plugin stage.  This way you
can add plugins while crawljax is running, so long as the stage where
the plugin will run hasn't started yet
- Reverted changes to JarRunner. The functionality is moved to
ClassLoaderHelper and PluginImporter
- specifically, PluginImporter now stores the directory where it will
look for plugins.
- ClassLoaderHelper will no longer attempt to enumerate non existing
directories.
- Removed the nullpointerexception expectancy from the
ImportFromNonExistentFolder test, since we no longer expect it to throw
an exception.