Add start date, end date and goal

[dahoo]

Lets the author define a goal (page or word count) and interpolate progress from start and to deadline.

Depends on #6.

[Kadrian]

If I reset the DB, insert a new paper, "start" and "goal" in paper.rb give back null values, and then Highcharts can't render it:
Error: Invalid value for <rect> attribute width="NaN"
Error: Invalid value for <rect> attribute x="NaN"

Here is the data["history"] array, any ideas?

history: Array[3]
  0: Object
    pages: 0
    time: null
    words: 0
    __proto__: Object
  1: Object
    pages: 1
    time: "2015-06-21T21:16:14.327Z"
    words: 257
    __proto__: Object
  2: Object
    pages: null
    time: null
    words: null
    __proto__: Object