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 see where you're coming from, however the value itself may be changed whenever, any number of times if they wish.  all caps is usually associated with an actual constant (i.e. API_KEY = "BLABLABLA" is never changed, ever).

Giving a mutable value all caps name will be confusing.