Added global Sift API key.  can be used as follows:

>>> import sift
>>> sift.api_key = '1234567890'
>>>
>>> client = sift.Client()
>>>
>>> client.api_key
'1234567890'
>>>
>>> client2 = sift.Client('0987654321')
>>> client2.api_key
'0987654321'
Added unit tests for new functionality.
Added one more clause to unit test.
enamed key to api_key for backwards compatibility.
updated error message in Client init.
Merge pull request #12 from SiftScience/yoav_client_libs_global_api_key

For Bernd: Python global api key