Search changes

This pull request adds the following changes:

 - Scrapped ElasticUtils/pyelasticsearch. Wagtailsearch now uses elasticsearch-py
 - Tests are no longer run with wagtailsearch signal handlers enabled
 - add_bulk is now a generator (so we can provide feedback during the bulk add process)
 - Indexed class refactor
 - Queries now run on QuerySets rather than models. This allows more complex filters to be run.
 - Refactored ElasticSearch backend. (better ElasticSearchResults class, more unit testable, indexes fields with correct type, partial_match fields are indexed separately from other fields).
 - Changed configuration format

TODO (for this PR):
 - [x] Unit tests
 - [x] Python 2.6 support
 - [x] Refactor/cleanup
 - [x] Testing

TODO (for future PR):
 - Remove wagtailcore specific code from search view
 - Split editors picks into own module (Done: #223)

Example of searching on a queryset (using PageQuerySet #185):

    events.get_descendants().not_descendant_of(past_events).live().search("Hello world!")

This will search all live pages that are in the events section but exclude pages which are in the past_events sub section.
I'm wondering if it might be worth changing ```search_fields``` to use a format more similar to setting edit handlers on your page.

For example:
```python
search_fields = {
    'name': dict(partial_match=True, boost=10),
    'biography': dict(),
}
search_filter_fields = ['age']
```

Becomes:

```python
search_fields = Page.search_fields + [ # Inheritance is no longer implicit
    SearchField('name', partial_match=True, boost=10),
    SearchField('biography'),
    FilterField('age'),
]
```

This also gives us syntax for not indexing a Page Type:
```python
search_fields = None
```