Reading cell and font colors done right

Hi Zeno, I'm working on adding a possibility to the gem to read cell background and font colors exactly how Excel displays them on screen. I've smoked the specs and added readers for some additional data that needs to be extracted to implement this feature. The branch is not finished yet, because obviously what we want from the gem is to output real color values, like '#add3f7', and the code to interpret this additional data is yet to be added (it is ready but is now in an external module).

If you think it's worth adding it to the gem, please comment on my reader implementation, (so this isn't really a pull request).