At last!

[RIFF][following size][WAVE]fmt[...]...data[data size][padded data].

Also, update example.wav.

This is specified in RIFF format, but I forgot to implement.

     ckData  Binary data of fixed or variable size. The start of
	     ckData is word-aligned with respect to the start of the
	     RIFF file.  If the chunk size is an odd number of bytes, a
	     pad byte with value zero is written after ckData.  Word
	     aligning improves access speed (for chunks resident in
	     memory) and maintains compatibility with EA IFF.  The
	     ckSize value does not include the pad byte.

(From http://www.kk.iij4u.or.jp/~kondo/wave/mpidata.txt)

Captcha ids are now 20 characters long.

This simplifies code and makes it faster. It also produces smaller PNGs.

Before:

captcha.BenchmarkNewImage	500	 6198230 ns/op
captcha.BenchmarkImageWriteTo	100	17517110 ns/op   0.14 MB/s

After:

captcha.BenchmarkNewImage	500	 3869968 ns/op
captcha.BenchmarkImageWriteTo	200	 7237604 ns/op   0.23 MB/s

(Core 2 Duo @ 2.26 GHz, MacBook)

Currently the background is white, not transparent, because PNG encoder
doesn't support paletted images with alpha channel.

Submitted CL: http://codereview.appspot.com/4432078

Change alpha to 0x00 once it's accepted to make captcha transparent.

Also, audio benchmark uses the new ioutil.Discard.

Instead, change level of individual reversed sounds when making
background to the appropriate values.

This speeds up makeBackgroundSound ~3%.

Before and after.
captcha.BenchmarkNewAudio      50  30436940 ns/op
captcha.BenchmarkNewAudio      50  29435780 ns/op

Also, level up white noise a bit.

Also:
- use ranges everywhere
- optimize makeWhiteNoise a bit

Before and after:
captcha.BenchmarkNewAudio      50  32128580 ns/op
captcha.BenchmarkNewAudio      50  30436940 ns/op

For the standard size and width with new font they no longer had any
effect anyway (were always +0).

NewLen is now like the old New (accepts captcha length as the argument).
Also, rename StdLength to DefaultLen.

Garbage collection is an internal detail of a Store, and requiring to
export this method seem to be not very useful.

The default memory store, of course, still has garbage collection, but
it's not the unexported method, called in Set as usual.

Use rndf instead of rand.Float64 voodoo.

Only return page for root path /, return 404 for any other, such as
/favicon.ico.

The server no does nothing for errors other than ErrNotFound (for which
it calls http.NotFound). This is becase when getting error on writing,
we already wrote headers, so can't respond with status 500.

Possibly log errors in future?

Also, refactor methods.