Rule: no-todo - disallow TODOs in source code

This fixes issue https://github.com/nzakas/eslint/issues/138. While the rule may not be particularly useful it would be interesting to allow the comments to be emitted through the API which is in this commit. This would allow for custom rules (ie not allowing code to leak the name of employees in comments or other sensitive information) to filter comments on `Block` and `Line` node types from esprima without having to search the raw source of the other nodes.