Standard storage is now known as memoryStore (created with
NewMemoryStore exported function). It's still default, however there's
now an option to replace it with a custom store by implementing Store
interface and calling SetCustomStore.

Fix adding extra space in the generated sounds.go.

Prevoiusly, Expiration and CollectNum were hard-coded into store.go.
This change moves these constants to captcha.go (renaming to Std*), and
makes newStore accept them as arguments.

This will enable setting these paraments by package users sometime in
the future, if needed, and also allows testing of the store garbage
collection.

Plus fix comments.

The new command replaces old test command-line utilities from cmd/image
and cmd/audio directories. It can now generate both image and audio
captchas, and accepts a few useful flags.

Edit .gitignore to reflect changes.

Audio test cmd now writes to stdout.

Shorten intervals between prelude beeps.