Extract piped migration into Redis::PipedMigrator
Add Redis::NativeMigrator

Uses Redis MIGRATE command to copy keys between redis instances.
Drawbacks:
  * Kind of nasty testing since MockRedis doesn't support MIGRATE
  * No 'do_not_remove' support since removing is part of MIGRATE
Automatically choose migrator

Depending on redis version in all the involved nodes and if keeping
the original data is required (MIGRATE with COPY only available since
Redis 3.0)
Merge pull request #4 from redross/master

Add NativeMigrator that uses Redis MIGRATE command