Openbazaar python launcher

new pull request after 1st round of reviews.
Thanks @Renelvon this has the merged changes from your repo.

(@Renelvon I kept that REVERT commit you put there as I'm not sure I understood what was right or wrong about it, I figured it'd be easier to fix here now)
util.is_mac() is a new utility method that does what your comment says 
```
def is_mac():
    os_name = platform.uname()[0]
    return os_name == 'Darwin'
```

as we move forward, we should have here other os checking methods in util, didn't want to implement them yet as they're not necessary and I didn't have a windows machine to check.