List registered commands in help text

Fixes #34
Closes #54

I should probably come up with a way of testing this. For the record, this changes `-h`'s output to:

```shell
~/sandbox/twine (bug/34) twine -h
usage: twine [-h] [--version] {upload}

positional arguments:
  {upload}

optional arguments:
  -h, --help  show this help message and exit
  --version   show program's version number and exit
```

Note the difference in that positional arguments will now be a list of possible commands