Fixes Issue #27

This fixes issues #27. If you try calling the get_domains() method on an account with no domains, an error is thrown.

```python
clouddns/connection.py", line 189, in list_domains_info
    for _offset in xrange(offset, offset + limit, step):
ValueError: xrange() arg 3 must not be zero
```

This commit checks to make sure step is set to something greater than 0, if not it will default to 1.