add commands for greping item

I've added the following commands for the incremental search of item.

* `direx:search-item` ... Do incremental search
* `direx:show-all-item-at-point` ... Turn back the hidden items under cursor.
* `direx:show-all-item` ... Turn back the hidden items in buffer.

![test-direx](https://f.cloud.github.com/assets/3935754/2133915/840599de-92d1-11e3-82be-b6b2a6240943.gif)

In addition, I've fixed the following problem.
When do `direx:jump-to-directory`,
If the target item, which is the value of `buffer-file-name`, is invisible,
`direx:goto-item-for-tree-1` can't move to that.

BTW, I'd gotten this architecture from anything.el.
It's the idle timer, which is started by `direx:search-item`, grep buffer in background using the search string that user input into minibuffer.

Best regards.