Allowing on-the-fly interval changing.

* Refactored the code a bit to support interval changing while the Speed Reader is still running (HTML5 `oninput` event rocks!).
* Minor stylistic changes (such as `null` instead of `undefined`, and adding `var` to global variable declarations).
* Stripping away leading and trailing whitespace from the input string.
* Splitting on any amount of whitespace, which means that multiple spaces won't generate empty tokens.
* Added a short pause (one-"interval" length) after the text is finished.

This entire pull request has been created using the web interface.