Store images on Amazon S3 (CollectionFS)

**Adds the ability to use CollectionFS + S3 to store your post images.**

 - Medium-insert, drag-and-drop, and progress bar will use S3 if enabled
 - Uses `Meteor.settings` instead of `Blog.config` for options because
   the code that sets up S3 loads before `Blog.config`.

**To setup S3 for file storage, add the following in `lib/settings.json`:**

```json
 {
   "public": {
     "blog": {
       "useS3": true
     }
   },
   "private": {
     "blog": {
       "s3Config": {
         "bucket": "you-bucket-name",
         "ACL": "public-read",
         "MaxTries": 2,
         "accessKeyId": "XXXXXXXXXXXXX",
         "secretAccessKey": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
       }
     }
   }
 }
 ```
 **Then, Run meteor with `meteor --settings=/lib/settings.json`**

**Note:** At this point we have configs in 3 different places (server, client, settings.json). Instead, we should probably use just `Meteor.settings` for all the config since it's guaranteed to be available by the time your app loads, and it would be secure.