Boosting loading speed of valid_email gem

Loading speed before

```
$ curl -fsSL https://gist.github.com/raw/5022636/benchmark.rb | ruby

Gem                            Time(sec)     Pct %
--------------------------------------------------
rails                             9.2416     44.6%
valid_email                       1.5996      7.7%
```

and after my improvements

```
valid_email                       0.0076      0.0%
```

See the difference? :) Also I've done bunch of useful changes. See pull request's commits for details.