Added reef-utils-hadoop

This adds a new sub-module to REEF for Hadoop/HDFS related utilities.
*Rationale:* Not all REEF applications depend on Hadoop and HDFS. Hence, adding these utilities to `reef-common` is impractical.

This also adds a first useful utility to this new project:
 * Added DFSHandler, a logging handler that writes straight to DFS
Merge pull request #776 from Microsoft-CISL/weimer_775

Added reef-utils-hadoop