Refactored implementation and organization of Python CLI tools.

The initial Python CLI 'mesos-ps' tool used curl in order to make
concurrent HTTP requests. This was a giant hack (one which even
required getting the maximum open file descriptor limit to not
fork/exec too many curl subprocesses). Instead of using curl we know
use futures and threads. The choice to use futures was (a) because it
naturally abstracts what we're trying to do and (b) it's "pro style"
for Python > 3.x. For now, we've implemented the futures interface
minimally to accomplish what we need (see
src/cli/python/mesos/futures.py). In addition, we updated the way
mesos-ps was implemented to leverage the futures API.

To enable sharing of CLI utilities written in Python we've also added
a mechanism for installing Python modules. This will likely change in
the future to use PIP but for now we manually update PYTHONPATH (see
bin/mesos.sh.in and src/cli/mesos.cpp).

Finally, all of the CLI specific Python code has been updated to use
the standard four space indent rather than two.

Review: https://reviews.apache.org/r/15260
Added mesos-cat and mesos-tail.

These are actually slightly improved version of the original shared by
Ben.H at mesos hackathon on Oct.25th 2013.

Modifications I did:
- add python version check
- add quote for query contents
- support to inspect files on completed frameworks/tasks

From: Shingo Omura <everpeace@gmail.com>
Review: https://reviews.apache.org/r/14951
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