Decodes input and encodes output

Currently cinderclient doesn't handle properly incoming and outgoing
encode / decode process. As a solution for this, this patch implements a
decoding process for all data incoming from the user side and encodes
everything going out of the client, i.e: http requests, prints, etc.

This patch introduces a new module (strutils.py) taken from
oslo-incubator in order to use 2 of the functions present in it:

About safe_(decode|encode):

    Both functions try to encode / decode the incoming text using the
    stdin encoding, fallback to python's default encoding if that
    returns None or to UTF-8 as the last option.

    In both functions only basestring objects are accepted and they both
    raise TypeError if an object of another type is passed.

About the general cinderclient changes:

    In order to better support non-ASCII characters, it is a good
    practice to use unicode interanlly and encode everything that has to
    go out. This patch aims to do that and introduces this behaviour in
    the client.

Testing:

    A good test (besides using tox) is to use cinder client with and
    without setting any locale (export LANG=).

Fixes bug: 1130572

Change-Id: Idb7d06954c29e003f68a0c4aa0b80ecc7017cbc9