Automatically detect MapR

This is an improved version of my last pull request; rather than using mrjob's config it tries to use the system config. There are a few caveats:

* I can't think of a way to test this, since we can't really shim the hadoop command
* The hadoop tests will fail if you run them on MapR, because the detection is working correctly
* The 'hadoop conf' command fails on CDH4; in this case we assume hdfs:// as the scheme. This is correct and works, but it might be improved.