Add store test.

Fix collection (locking and getting next value).

newStore: accept collectNum and expiration time.

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.

Add ErrNotFound, return it from WriteAudio/Image.

Export RandomDigits. Remove NewRandom* functions.

Rename "numbers" to "digits" everywhere.

Plus fix comments.

Set capacity for audio buffer. Rename makeStaticNoise to makeWhiteNoise.

Rearrange code and fix docs.

Implement a single captcha test command.

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.

Add NewRandomAudio function.

Audio test cmd now writes to stdout.

Replace PNGEncode with WriteTo for Image.

Add WriteAudio function.

Rename CaptchaImage -> Image, CaptchaAudio -> Audio.

Apply gofmt; fix comment in generate.go.

Better randomize background and sounds.

Shorten intervals between prelude beeps.

Lower level of beep sound.

First version of audio captcha.

Move image generation test program to cmd/image.

image: move border calculation to calculateSizes().

Fix comments.

Move rand initialization to image.go.

Clarify docs.

image: refactor; add more color variations.

Refactor store into its own file.

Accept the length of the random sequence of numbers.

Call Collect on a separate goroutine.

Update documentation.

cmd: use standard width and height for image.

Document functions and types in captcha.go.

Refactored image captcha ready, and documented.

Numbers are now laid out correctly.

Next commits will introduce perturbations.

Started work on code cleanup.

Gofmt font.go.

Move image-related functions into image.go.

Add line that strikes through digits.

Reorganize code. Add storage.

Initial version (numbers).