Add functionality to filter by last name

  * Click on a letter at the top, and the people with last
    names starting with that letter will be displayed
  * like a rolodex!
  * but the code looks terrible. refactor coming up!
Refactor search, render, and filter events

  * filter, search, and render work a bit differently now
  * render receives an optional filter object, which
    contains a search callback and search term
  * in render():
    * all views are cleared
    * the collection is sorted alphabetically
    * if there is a callback/search term passed, renderSome is called
    * if there is no callback, render all is called
    * renderSome checks each contact against the search term
      * if it matches, it's rendered
      * if it does not match, it's ignored
    * renderAll goes through each contact and renders it
Merge pull request #13 from mknudsen01/filter-last-name

Filter last name