support for SORT
use key param to support instead of cmp for compatibility across python versions
Add clock abstraction instead of embedding datetime.now()

Improve unit tests to compare expected ttl against ttl with delta.
Merge branch 'support_sort' of git://github.com/shearic/mockredis into shearic-support_sort
Formatting, flake8, and py3 fixes for sort()
Implement correct usage of alpha in sort()