Couple of fixes and improvements

[vesse]

• Added missing require('fs') (fixes #1)
• Use req.set if it exists, otherwise use req.setHeader (support new nodejs versions)
• Added assetFingerprint to global context so it actually works from Jade
• Second parameter to constructor turned into an options hash with the following possible options:
  • cache: the cache strategy object (formerly the only optional parameter)
  • debug: turn on the (only) console.log line
  • set_expires_headers: set to false to disable Expires and Cache-Control headers

[bminer]

Many of these improvements were added to the latest version 0.4.2. If you want to request additional functionality, please open a separate issue. Thanks for your help and for posting this pull request!