add zip_safe flag to setup, to silence multitude of warnings

without the `zip_safe` flag set to `False`, a whole bunch of warnings are spit out when installing easybuild-easyblocks

```
zip_safe flag not set; analyzing archive contents...
easybuild.__init__: module references __path__
easybuild.easyblocks.__init__: module references __file__
easybuild.easyblocks.__init__: module references __path__
easybuild.easyblocks.generic.__init__: module references __path__
```

see also http://stackoverflow.com/questions/8362510/getting-rid-of-the-easy-install-message-module-references-file