Feature: Add SelectMultipleListView

I needed to implement this for another project and thought it might come in handy for other people aswell. If you like the idea I can add specs.

![gifrecord_2014-06-08_112859](https://cloud.githubusercontent.com/assets/5219415/3210912/6d56ce0e-eeef-11e3-9ad5-e1c76720c069.gif)

#### Example:
```coffee
{$, $$, SelectMultipleListView} = require 'atom'

module.exports =
class TestMultipleListView extends SelectMultipleListView

  initialize: ->
    super
    @addClass 'overlay from-top'
    @parseData()

  parseData: ->
    items = [
      {id: 1, human: true, male: 'not'}
      {id: 2, human: false, male: 'not'}
      {id: 3, human: true, male: 'yes'}
    ]

    @setItems items
    atom.workspaceView.append this
    @focusFilterEditor()

  viewForItem: ({id, human, male}) ->
    $$ ->
      @li "#{id}, #{human}, #{male}"

  completed: (items) ->
    str = ''
    str += "[#{id}, #{human}, #{male}]\n" for {id, human, male} in items
    atom.confirm
      message: 'You selected:'
      detailedMessage: str
    @cancel()
```