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'
```
Use ALL CAPS for global / class constants.

`API_KEY`

https://google-styleguide.googlecode.com/svn/trunk/pyguide.html?showone=Naming#Naming