Update search to use V1.1 API methods and results

Initial changes required to make `search` work with the Twitter V1 API.   Please refer to the Twitter [1.1 API search/tweets](https://dev.twitter.com/docs/api/1.1/get/search/tweets) documentation and [errata](https://dev.twitter.com/issues/513) for implementation details.

Please review and comment.  This works in my limited testing, but I cannot run the full test suite in my environment. 

- search() fix path (URL) to use 1.1 API url
- search() allowed_param changes: rpp -> count, adding include_entities; 
  allowed_param entries now in alphabetical order.
- search_api flag no longer needed or used.
- SearchResults class: Pull list results from ['statuses'], use
  parse_datetime() rather than parse_search_datetime() since the date
  format is now standardized.
- utils.parse_search_datetime() no longer used and has been deleted.

### Cursor support ###

`search` does not support reverse Cursor pagination at this time.  

### Breaking changes ###

* The parsed `SearchResult` structures have changed due to changes in the underlying API. `models.SearchResult` is now a subclass of `models.Status` -- the results list is now parsed as a list of `models.Status` objects, and the search *metadata* is contained in the `search_metadata` attribute of `API.search()`'s returned value.