.gitignore: Add std{out,err}_logfile

Just add 2 files to `.gitignore` that get created by the tests.

```
$ ls -l stderr_logfile stdout_logfile
ls: stderr_logfile: No such file or directory
ls: stdout_logfile: No such file or directory

$ tox -e py26
...
  py26: commands succeeded
  congratulations :)

$ ls -l stderr_logfile stdout_logfile
-rw-r--r--  1 marca  staff  0 Feb 21 17:43 stderr_logfile
-rw-r--r--  1 marca  staff  0 Feb 21 17:43 stdout_logfile
```