Merge pull request #381 from joshuaflanagan/scan_return_vals

Improve return values for zscan and hscan
DRY up reply transformation.
Refactor `_scan` so that it doesn't need to check for its caller.
scan_each commands to wrap *SCAN cursor-based calls

The *scan_each commands can all take a block, and will yield each value
to the block.
If no block is provided, an Enumerator is returned.