Make sure `test/db` is always available.

Closes #380.
Expose SCAN command as an Enumerator
Expose HSCAN command as an Enumerator (#hscan_each)
Expose ZSCAN command as an Enumerator (#zscan_each)
Expose SSCAN command as an Enumerator (#sscan_each)
Only run *scan_each tests on redis > 2.7.105