Add nickname as parameter to register decorator

Latest version of Swagger UI does not seem to accept empty nicknames in operations. I added a keyword argument 'nickname' to register function to be able to specify non null nicknames. 

With the example from the README, it would be used as follows:
```python
@register("/api/v1/cheese/<cheeseName>",
  parameters=[
    ApiParameter(
        name="cheeseName",
        description="The name of the cheese to fetch",
        required=True,
        dataType="str",
        paramType="path",
        allowMultiple=False)
  ],
  nickname="cheeseNickName",
  responseMessages=[
    ApiErrorResponse(400, "Sorry, we're fresh out of that cheese.")
  ])
```

Related to #2.