Add support for memory usage ... command

[jasonpenny]

MEMORY USAGE key currently results in NoMethodError: undefined method 'memory' for #<MockRedis::Database...

This change will return a hardcoded numeric value

> m = MockRedis.new
> m.call(["memory", "usage", "foo"])
=> nil

> m.set("foo", "anything")
=> "OK"

> m.call(["memory", "usage", "foo"])
=> 160

[sds]

Thanks!