reword introduction

  - Underscore is now more widely recognized than Prototype.js, so describing the former as resembling the latter is not as helpful as it once was.

  - We currently describe Underscore as both a tool-belt and a tie. :)

  - I dropped the comma after "tux". The comma doesn't seem right to me.

  - "Backbone.js's suspenders" is awkward. People refer to the library as Backbone in conversation, so dropping the suffix in this sentence seems sensible.