Improve the input method behavior

This change makes the behavior of the input method close to the one on the textarea element.

CodeMirror does not show the current input candidate on the editor (fig1).
"こんにちは" is the raw input and "今日は" is selected as the conversion result.
This pull request changes the behavior as shown in fig2.
This behavior is much closer to the text input on the textarea element and less frustrating (at least to me).

I tested this only on MacOS and with a Japanese text input method (Google IME).
So I'm not sure this change works for the other environments and languages too.

fig1:
![before](https://cloud.githubusercontent.com/assets/113428/2686640/2ad5732c-c213-11e3-9541-3632fcfe811e.png)

fig2:
![after](https://cloud.githubusercontent.com/assets/113428/2686639/2ac9e9c6-c213-11e3-9de0-9bd8845ee35a.png)


Thank you.