Accept dict in SavedQueriesInterface.create()

[masojus]

Accept a dict for the saved_query properties, and convert to a json-formatted str to send as the PUT payload. Fixes Issue #119

• This is technically a breaking change since, if client code is using this and working around it by calling my_keen_client.saved_queries.create('new_query_name', json.dumps(new_query_attributes)), then this will no longer work.
• This code has been in the repo for a long time, so I'm not sure how to handle this version-wise.

[masojus]

It also looks like one can use the Requests "json" param instead of the "data", but we don't do that elsewhere: http://docs.python-requests.org/en/master/user/quickstart/#more-complicated-post-requests