Improved Feed

Can now be unserialized by Mapper
Now using Feed._items instead of API if possible for less API calls
Support for since/before parameters (fix #100)

Examples:
Using the items method:
  Return objects older than obj, newest first
    Feed.items(before=obj_or_id, limit=None_or_int)

  Return objects newer than obj, oldest first
    Feed.items(since=obj_or_id, limit=None_or_int)

  Return objects starting from offset, newest first
    Feed.items(offset=None_or_int, limit=None_or_int)

Using slicing:
  by index:
    Feed[None_or_int:None_or_int]
  by id:
    Feed[obj_or_id:] or Feed[:obj_or_id]
  get item by index:
    Feed[int]