Add ediff faces

Use lighter background for all the diffed hunks in ediff. Color the
background of the selected hunk in red, gree, and blue for the old, new,
and merged, respectively.

See the following screenshots for the comparision.
![ediff-original](https://f.cloud.github.com/assets/102858/1061811/09238d88-1208-11e3-9592-3afe2ae4e6a7.png)
![ediff-zenburn](https://f.cloud.github.com/assets/102858/1061812/09241a00-1208-11e3-812f-bb6612e2d342.png)