Use specified rather than computed values. Fixes #8

* specified_values:
  Update tests to reflect new way of calculating
  Avoid unsavory nodes inside head tag
  Abbreviate background properties
  Bookmarklet should start at document head
  Zero is falsey, so check for undefined explicitly
  Start style search all the way at html tag
  Point bookmarklet to experimental code
  Use specified rather than computed property values
  Use _.pick where possible