Blank out whitespace in popups.

I am using `whitespace-mode` to visualize whitespace:

```
$ emacs -Q
```

```elisp
(whitespace-mode 1)
(add-to-list 'load-path "~/.emacs.d/company-mode")
(autoload 'company-mode "company" nil t)
(company-mode 1)
(def
```

At this point, the `company-mode` completion popup apppears. Since its background is drawn using spaces, they get affected by `whitespace-mode` and displayed as middle dots.

In normal editing they are fontified with a face which assigns them a very subtle color, however in `company-mode` popups they seem to be displayed in normal popup face. This is distracting.

![before](https://cloud.githubusercontent.com/assets/238691/5234630/a335edfe-77ff-11e4-8576-6a0aa080b066.png)

In this patch, I force whitespace in `company-mode`’s popups to have foreground identical to the background, so whitespace is invisible.

![after](https://cloud.githubusercontent.com/assets/238691/5234637/17247e88-7800-11e4-96d3-cd1ea3b8b596.png)

(This may require tweaking also the `:distant-foreground` face attribute on Emacs 24.5 — I haven’t yet had the chance to upgrade and so can’t test.)