New feature: silent (human) test result.

Implement new option --silent, to mute the human test result.
It has no effect with --json or --xunit options.

Example:

$ avocado --silent run failtest
$ echo $?
$ 1

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #141 from lmr/silent_output

New feature: silent (human) test result.