Allow global=>0 as an option to the Memory driver.

I like to declare my CHI configuration in static configuration files.  This has issues with the Memory driver in particular as I almost always want a local datastore.  But, the only way to declare a local datastore is by passing a hashref.  I can't create a new hashref in the configuration, that just doesn't make sense.  The alternative I have is to have code that injects a new hashref after loading the config, but before calling CHI->new(), and thats just cumbersome.

The way I thought it would work, but turns out it doesn't, is by passing global=>0, implying no globalness.  Since that doesn't work, I implemented it.

I hope you like.  Let me know if you want any changes.