Add support for redis-py 3+

[YuriHeupa]

redis-py 3.X introduced some breaking changes that made this project incompatible, this PR adds support for these changes and fix tests were applicabble. Follows some redis-py changes:

https://github.com/andymccurdy/redis-py#mset-msetnx-and-zadd

MSET, MSETNX and ZADD
These commands all accept a mapping of key/value pairs. In redis-py 2.X this mapping could be specified as *args or as **kwargs. Both of these styles caused issues when Redis introduced optional flags to ZADD. Relying on *args caused issues with the optional argument order, especially in Python 2.7. Relying on **kwargs caused potential collision issues of user keys with the argument names in the method signature.

https://github.com/andymccurdy/redis-py#encoding-of-user-input

redis-py 3.0 only accepts user data as bytes, strings or numbers (ints, longs and floats). Attempting to specify a key or a value as any other type will raise a DataError exception.

https://github.com/andymccurdy/redis-py#zincrby

redis-py 2.X accidentily modified the argument order of ZINCRBY, swapping the order of value and amount

So this PR change all ZADD calls to supply keys and values as a dict instead of positional arguments, and it also update tests to provide member_data as string instead of dict, as stated in docstring methods, these data should be provided in string already.

Fixes #61

[YuriHeupa]

Btw, it also fixes a typo on this function.

[YuriHeupa]

Rebased and resolved conflicts.

[tito]

Could this be packaged in a new version ? Just pip install redis breaks usage of leaderboard

[maxifom]

+1