incremental release

Changes:

• Dropped following Redis method -- Redis.multi(), Redis.exec(), Redis.discard()
• Redis.multi_exec hack'ish property removed
• Redis.multi_exec() method added
• High-level commands implemented:
  • generic commands (tests);
  • transactions commands (api stabilization).
• Backward incompatibilities:
  • Following sorted set commands' API changed:
    zcount, zrangebyscore, zremrangebyscore, zrevrangebyscore;
  • set string command' API changed;