first batch of tests, mostly borrowed from redis-py, adapted to cluster specific implementation.
Merge pull request #11 from happybits/tests

first batch of tests
get_connection_by_key() should raise RedisClusterException not Exception
Updates to tests

 - Wrote how to test in README
 - Minor pep8 in conftest.py
 - Added one new fixture in conftest.py for tests that do not require to init the slots cache
 - Added tests for RedisCluster class. (TestClusterObj class)
 - Added 2 more tests for pipeline testing
Added authors document