Make default shellCacheDir on Linux/OS X

I don't see much need to clutter people's home directory with the moz-commitbuilder-cache so I suggest that it is prefixed with a '.' so that it is hidden on Linux/OS X.  This patch provides a migration path for existing users by first checking if ~/moz-commitbuilder-cache exists and using that. Otherwise, it defaults to ~/.moz-commitbuilder-cache for shellCacheDir.