Update assets resources for high display resolution

It updates assets resources for high display resolution such as Retina Display. This is the take 2 of #294.

* New `favicon.ico` contains 2 sizes icons (16 px and 32 px) made by ImageMagick.
* Larger YUI logo image for `default` and `simple` themes, and user guide page (same with yuilibrary.com).

`yuidoc.json` still be able to accept only one `logo` configuration. It should be accept multiple configurations, but I'd like to choose another time to consider it.

/cc @juandopazo 

## Before

![2014-12-10 at 22 16](https://cloud.githubusercontent.com/assets/34588/5411657/2ffbfa12-8246-11e4-9b83-7bf6b27b3c7f.png)
## After

![2014-12-10 at 22 11](https://cloud.githubusercontent.com/assets/34588/5411653/1e243f34-8246-11e4-9144-7e39d3144433.png)
![2014-12-10 at 22 35](https://cloud.githubusercontent.com/assets/34588/5411659/35f3dcf0-8246-11e4-9c45-d944436a316a.png)