Improved documentation and a test

[jlangson]

Hey Dan, I hope you consider this a belated Christmas present.

• Found links for most of the papers and added them to the readme and com.dean.interval-tree.tree.tree.
  • Fixed broken Oleg link.
• Added a property test that checks tree health after adding or removing nodes.
• There's a new method for choosing a random-node. I don't know how useful that would be in production, but it's helpful for testing.
• Added a comment regarding the normalize function.
• Some spelling corrections.

Let me know if you want me to rebase this.

[jlangson]

Fixed whitespace issue.