add the ability to create hdfs symlinks

I wrote this PR before I realized that in most versions of hdfs (pre-2.1.0, according to [HADOOP-8040])(https://issues.apache.org/jira/browse/HADOOP-8040), *reading* via symlinks is not supported. So while creating symlinks seems to work fine, the tests fail because the namenode throws an `UnresolvedPathException` when you try to `ls` the symlink or do anything useful with it.

I realize that makes this feature next-to-useless, but since I already wrote it I figured I'd send it over just in case you wanted the code (it should work fine with cdh5, I figure). Feel free to close this PR if you don't.