Optimise images (95 KB reduction)

Hi, fnando.

I've taken the liberty of putting kitabu's image assets though a range of optimisation tools. Image optimisation allows us to reduce the footprint of images - meaning faster load times at no cost to image quality. It does this by finding the best (lossless) compression parameters, stripping embedded comments, and removing unnecessary colour profiles. [See more](http://port3000.co.uk/imageoptimiser-a-github-bot-to-proactively-op).

I found 6 optimisable images, making a 95 KB reduction in size. That's about 50.00% of your total image assets.

Merry Christmas.

<table><tr><th>Name</th><th>Reduction</th></tr><tr><td>challenge.png</td><td>25.81%</td></tr><tr><td>posts_index.png</td><td>96.10%</td></tr><tr><td>rails_welcome.png</td><td>30.98%</td></tr><tr><td>logo.gif</td><td>0.09%</td></tr><tr><td>cover.png</td><td>59.66%</td></tr><tr><td>ebook.png</td><td>39.81%</td></tr></tr></table>

Tools used: advpng, gifsicle, jpegoptim, jpegrescan, jpegtran, optipng, pngcrush, pngout. This is an automated bot maintained by [@skattyadz](https://twitter.com/skattyadz)