This brings a change in behaviour, in which the stdout/stderr of the
executed process will now be of bytes type, instead of a string type.

Two new attributes, which are implemented as properties, have been
added to the CmdResult class, `stdout_text` and `stderr_text`.  Those
are convenience methods that will return the same content that is in
`stdout` and `stderr`, reespectively, but decoded on the fly[1].

With regards to encoding, if one is not provided, the result of
`sys.getdefaultencoding()` will be used ("utf-8" for Python 3 and
"ascii" for Python 2).

Applications and/or tests using the APIs that return a CmdResult
should, to the best of my knowledge, set a default encoding themselves
so a stable behavior across Python versions.  But that if left to
users of this API.

A different tradeoff/design decision has to do with the tests modified
here.  One option is to have "text" (as in sequences of human readable
glyphs) as being of Python type "str".  On Python 2, "str" can be
compared to "bytes" because a conversion will happen on demand.  That
is, the following is fine on Python 2:

   >>> result = process.run("command")
   >>> "expected" in process.stdout

Where `expected` is of type "str" and `process.stdout` is of type
"bytes".  This is not true of Python 3, so either the types must match
or a conversion must be done explicitly.  The solutions to that are:

1) have these "text" as (of type) "bytes" in the source code itself,
   and avoid the conversion whenever possible
2) have "strings" in the source code itself, and use the conversion
   provided by `CmdResult.stdout_text` and `CmdResult.stderr_text`.

The approach chosen here is to avoid conversion if possible, that is,
use "byte" types, given the fact that the source code encoding is by
default 'ascii' and most of the "text" dealt with here can be
represented in 'ascii' too.  This is equivalent of doing:

   result = process.run("command")
   b"expected" in process.stdout
   "errors: %s" % 0 in process.stderr_text

[1] The obvious alternative, instead of decoding these on the fly
    would be to have multiple copies of the "same" data.  This assumes
    that binary data produced on the stdout/stderr will usually be
    larger than textual data.
Signed-off-by: NCleber Rosa <crosa@redhat.com>