Add HADOOP_LIBRARY_PATH config setting so Hadoop will include external directories for jni

Currently there is no way to configure Hadoop to use external JNI directories. I propose we add a new variable like HADOOP_CLASS_PATH that is added to the JAVA_LIBRARY_PATH before the process is run.

Now the users can set environment variables using mapred.child.env. They can do the following
X=Y : set X to Y
X=$X:Y : Append Y to X (which should be taken from the tasktracker)