Included mesos-cat and mesos-tail in installation.

Review: https://reviews.apache.org/r/15299
Created 'usage' and moved 'get' helper to cli.py.

Review: https://reviews.apache.org/r/15300
Refactored mesos-cat to use helpers from cli.py.

Review: https://reviews.apache.org/r/15301
Revised os::realpath to return Result for when path does not exist.

Review: https://reviews.apache.org/r/15302