Add support for binary zipfiles as results from calls to RetrieveExport.

[TheRealRainerHock]

Calls to https://api.kraken.com/0/private/RetrieveExport return binary zip archive containing the report. They are returned as a ZipFile.

Additionally plain text (according to the content-type of the result's headers) isn't treated as json either.

[veox]

As mentioned in issue #114, this makes the return type of _query() (and, even worse, query_{private,public}()) vary depending on provided function arguments.

This approach is wholly unacceptable for me - it's ugly and results in bad documentation.

[TheRealRainerHock]

Sounds reasonable, but at the moment RetrieveExport is the only way to retrieve informations about staking and unstaking funds and the workaround you mentioned in #114 also has flaws. Would you accept a new method def query_private_zip(self, method, data=None, timeout=None)?

[veox]

Personally, I wouldn't care if the issue hangs indefinitely.

The way I see it, the issue is upstream at Kraken, with their dismal design choice regarding this particular query.

Would you accept a new method def query_private_zip(self, method, data=None, timeout=None)?

I've long resorted to keeping such methods of niche utility in a personal-use ExtendedAPI class, derived from krakenex.API.

What is definitely acceptable is an example of such an approach that would go in the examples directory.

[veox]

I've added just such an example in examples/retrieve-csv-report.py.