Improvement handling of multibyte characters

When we type multibyte chalacters, as follows:

```
$ ruby repl.rb
>> "abcdef"
=> "abcdef"
>> "あ                                                                                                                                       >> "あい                                                                                                                                     >> "あいう                                                                                                                                   >> "あいうえ                                                                                                                                 >> "あいうえお                                                                                                                               >> "あいうえお"
=> "あいうえお"
>>
```
![screen shot 2014-08-31 at 9 30 43 am](https://cloud.githubusercontent.com/assets/950573/4100619/11962b52-30a6-11e4-8bab-9049561aec3e.png)


This PR makes like this:

```
$ ruby repl.rb
>> "abcdef"
=> "abcdef"
>> "あいうえお"
=> "あいうえお"
>>
```