Adding a more elaborate and informative README

First of all, let me just say what a wonderful library this is - thank you for open sourcing it!

The README, as it is, is rather poor and not very informative. So, I used the information available over at http://github.hubspot.com/pace/ and put together a more elaborate README so that people looking at pace can get some context as to what it is all about and how insanely easy it is to use.

Any feedback on my suggested changes is welcome.

Andreas