For Bernd: Python global api key

@berndverst 

Added a global API key that can be used by all Client instances in scope of a given `import sift`.

Usage:
```
>>> import sift
>>> sift.api_key = '1234567890'
>>>
>>> client = sift.Client()
>>>
>>> client.api_key
'1234567890'
>>>
>>> client2 = sift.Client('0987654321')
>>> client2.api_key
'0987654321'
```
I'd argue that it is essentially an uninitialized constant. Once you set it, it is constant.