Only call chmod if perms > 0

Sometimes perms end up being 0 for some unknown reason. However this used to work fine...

```
$ omego download source
2014-12-05 15:09:36,461 [omego.artifa] INFO  Checking http://ci.openmicroscopy.org/job/OMERO-5.0-latest/ICE=3.5,jdk=8_LATEST,label=octopus/183/artifact/src/target/openmicroscopy-5.0.6-10-213b2a3.zip
2014-12-05 15:09:36,461 [omego.fileut] INFO  Downloading http://ci.openmicroscopy.org/job/OMERO-5.0-latest/ICE=3.5,jdk=8_LATEST,label=octopus/183/artifact/src/target/openmicroscopy-5.0.6-10-213b2a3.zip
2014-12-05 15:09:37,897 [omego.artifa] INFO  Unzipping openmicroscopy-5.0.6-10-213b2a3.zip
2014-12-05 15:09:38,828 [omego.artifa] ERROR Unzip failed: [Errno 13] Permission denied: 'openmicroscopy-5.0.6-10-213b2a3/.classpath-template'
[Errno 13] Permission denied: 'openmicroscopy-5.0.6-10-213b2a3/.classpath-template'
ERROR: Unzip failed, try unzipping manually
```

```
$ ls -ld openmicroscopy-5.0.6-10-213b2a3/
d---------  2 simon  staff  68  5 Dec 15:09 openmicroscopy-5.0.6-10-213b2a3/
```
Checking in Python zipfile `.external_attr >> 16 == 0` which corresponds to chmod 000