Fix uploader_id exception and os.path.join() in Windows

On my Win7, Python 2.7 system ```Musicmanager.login()``` throws ```OSError: a valid MAC could not be determined. Provide uploader_id (and be sure to provide the same one on future runs).```
For some reason ```uuid.getnode()``` can't acquire the real HW address and thus returns a fake one, which Musicmanager doesn't accept. This adds a command line option to specify that uploader_id.

Another important change is the missing extra ```os.sep``` in ```os.path.join()``` in ```do_download()``` function. From the docs: ```Note that on Windows, since there is a current directory for each drive, os.path.join("c:", "foo") represents a path relative to the current directory on drive C: (c:foo), not c:\foo.```

Others are small tweaks.