added dry_run parameter. fixes #207

```
>>> from pyqi.util import pyqi_system_call

>>> pyqi_system_call('ls -la')
('total 64\ndrwxr-xr-x   16 caporaso  staff    544 Dec  2 11:32 .\ndrwxr-xr-x  310 caporaso  staff  10540 Dec  2 10:13 ..\ndrwxr-xr-x   16 caporaso  staff    544 Dec  2 11:40 .git\n-rw-r--r--    1 caporaso  staff     54 Aug  3 13:25 .gitattributes\n-rw-r--r--    1 caporaso  staff    265 Aug 26 12:59 .gitignore\n-rw-r--r--@   1 caporaso  staff   2071 Aug 26 12:59 COPYING.txt\n-rw-r--r--    1 caporaso  staff    232 Dec  2 11:32 ChangeLog.md\n-rw-r--r--    1 caporaso  staff    177 Oct 30 11:29 MANIFEST.in\n-rw-r--r--@   1 caporaso  staff   1260 Aug 26 12:59 README.md\ndrwxr-xr-x    4 caporaso  staff    136 Aug  4 13:48 build\ndrwxr-xr-x   11 caporaso  staff    374 Oct  4 10:11 doc\ndrwxr-xr-x   12 caporaso  staff    408 Dec  2 11:41 pyqi\ndrwxr-xr-x    3 caporaso  staff    102 Oct  4 10:11 scripts\n-rw-r--r--    1 caporaso  staff   3246 Dec  2 11:32 setup.py\ndrwxr-xr-x    5 caporaso  staff    170 Aug  6 07:16 tests\n-rw-r--r--    1 caporaso  staff     67 Oct 30 11:29 tox.ini\n', '', 0)

>>> pyqi_system_call('ls -la',dry_run=True)
ls -la
('', '', 0)

>>>
```