Colorize mode-line depending on Flycheck status.

Hi Sebastian,

This PR adds support for mode-line colorization depending on the Flycheck status.
It is possible to prevent to change the mode-line color by setting the variable `flycheck-face-inhibit-mode-line-updates` to not nil.

Here is the results on my mode-line:
![flycheck-mode-line-error](https://f.cloud.github.com/assets/1243537/400569/c01a0f6c-a8ae-11e2-95bf-00154a6645fb.png)
![flycheck-mode-line-warning](https://f.cloud.github.com/assets/1243537/400570/c025848c-a8ae-11e2-876c-80fab44695ab.png)

Cheers,
syl20bnr