jcr2vfs: add basic test for JCR export
jcr2vfs: do not use System.exit() in run() method

 * this method is used in tests so it should not
   kill the whole JVM.
jcr2vfs: throw an exception when the export fails

 * exception should be thrown to indicate that the
   export actually failed. Printing the stacktrace
   is not enough as it can't be easily detected by
   caller.
Merge pull request #4 from psiroky/export-test

Simple JCR export test + few other enhancements