Fix normalize_ja more natural

For natural Japanese text, full-width/half-width translation should be more complex.
I changed converter rule more suitable. This behavior is close to PHP's one which oftenly used by Japanese users.

```php
$expected = '「NodeJS」の0.11(.0)が、ベータ・リリースされます〜。~>';  // beautiful form
$dirty = '「ＮｏｄｅJS｣の０．１１（.０)が､ベータ･ﾘﾘｰｽされます〜｡~＞';  // dirty input
echo mb_convert_kana($dirty, 'KVa', 'utf-8') == $expected; // --> true!
```

See 1st example of http://php.net/manual/en/function.mb-convert-kana.php

Totally we Japanese prefer half-width symbols than full-width as much as possible. But some of half-width punctuations (`。、・`) are not supported by our domestic encodings: Shift_JIS, EUC-JP and ISO-2022-JP.

Full-width `〜` should never converted. It has different semantic to half-width `~`. PHP also do nothing to `~`.