Decode input and encode output

Currently glanceclient doesn't support non-ASCII characters for images
names and properties (names and values as well). This patch introduces 2
functions (utils.py) that will help encoding and decoding strings in a
more "secure" way.

About the ensure_(str|unicode) functions:

    They both try to use first the encoding used in stdin (or python's
    default encoding if that's None) and fallback to utf-8 if those
    encodings fail to decode a given text.

About the changes in glanceclient:

    The major change is that all inputs will be decoded and will kept as
    such inside the client's functions and will then be encoded before
    being printed / sent out the client.

    There are other small changes, all related to encoding to str,
    around in order to avoid fails during some conversions. i.e: quoting
    url encoded parameters.

Fixes bug: 1061150

Change-Id: I5c3ea93a716edfe284d19f6291d4e36028f91eb2
Merge "Decode input and encode output"