Support custom PyPI repositories

This pull request adds an additional `repository` option to the `pypi` publish plugin, to allow you to specify the URL to a custom PyPI compatible package index such as something like https://pypi.python.org/pypi/pypiserver

Sample usage looks like:

```
publish:
  pypi:
    username: {{pypi_user}}
    password: {{pypi_pass}}
    repository: http://pypi.example.org:8080
```

Additionally there appears to have been some reverse logic in the if statement in `deployCmd` where `-z` was used to validate the existence of a string.  I believe that `-n` was the intended test to use there, since `-z` tests if a string is empty and `-n` tests that it is not empty.  As such, the plugin does not work without this change.