TestResult pluggable with XUnit test result.

- TestResult pluggable.
- XUnit test results.
- Add attribute for plugin name.
- Add attribute for plugin status (enabled/disable)

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado.plugins: PEP8 fixes.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #14 from ruda/test_result_plugin

TestResult plugable with XUnit test result.