FEEDS: add limit params to fetch verb

[silverbucket]

when fetching data, it would be nice to have limitation options:

verb: 'fetch',
object: {
  limit: 10, // number of entries to return
  afterDateNum: 1300200, // date number to compare as cutoff
  afterId: '...' // a string that can be interpreted by the platform as to what to match it to, but it can be used instead of fromDateNum.
  beforeDateNum: " ",
  beforeId: " "
}

[silverbucket]

/**
   * Function: fetch
   *
   * Fetches feeds from specified source.
   *
   * Parameters:
   *
   *   job - Activity streams object containing job data:
   *         {
   *           actor: {
   *             address: 'feeds'
   *           },
   *           verb: "fetch",
   *           target: [
   *             {
   *               address: '[feed_url]'
   *             },
   *             ...
   *           ],
   *           object: {
   *             limit: 10,    // default 10
   *             from: 'after' // returns articles *after* the date string
   *                           // other valid value is 'before'
   *             date: 'Tue Nov 26 2013 02:11:59 GMT+0100 (CET)',
   *             // ... OR ...
   *             link: 'http://www.news.com/articles/man-eats-car',
   *           }
   *         }
   *
   *         Without any properties specified, the platform will return most
   *         recent 20 articles fetched from the feed.
   *
   *         `date` - can be a date string, or seconds since 1970.
   *                if set to 0, then the platform will return either the
   *                most recent (from: 'before') or the oldest (from: 'after')
   *                up to `limit`;
   *
   * Returns:
   *
   *   Sends back an message with an AS object for each article, and upon
   *   completion it will send back a response to the original request with a
   *   complete list of URLs in the feed and total count.
   *
   */