point the documentation and gemspec at Pearson's info

Hey Toby, I updated the README.md file to point to your repository instead of my fork.  I also edited the license.txt and pearson_sdk.gemspec to point at your info instead of mine.

If you're curious, this is the demo that I'm working on: http://www.beelinereader.com/demos/pearson (more info at http://www.beelinereader.com)