Add the ability to set the format in the decorator

Geckoboard has deprecated the parameter passing (although it can still be enabled). This change allows you to set
the response format like this:

```python
@NumberWidget(format='json')
def my_view_fun():
  ...
```
Doc strings should be after the function definition
Document new format options
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Merge branch 'feature/add_format_parameter' of https://github.com/geckoboard/django-geckoboard into geckoboard-feature/add_format_parameter

Conflicts:
	django_geckoboard/decorators.py
	django_geckoboard/tests/test_decorators.py
Import setuptools to support extras_require
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