Added support for text search and text_score.

I wrote a support to $text search and textScore to documents.

```python
class News(Document):
    title = StringField()
    content = StringField()
    is_active = BooleanField()

    meta = {'indexes': [
          {'fields': ['$title', "$content"],
           'default_language': 'english',
           'weight': {'title': 10, 'content': 2}
          }
    ]}

News(title="Using mongodb text search",
       content="Testing text search").save()

News(title="MongoEngine 0.9 released",
       content="Various improvements").save()

document = News.objects.search_text('testing', include_text_score=True).first()
document.title # may be: "Using mongodb text search"
document.text_score # may be a float number
  
document = News.objects.search_text('released').first()
document.title # may be: "MongoEngine 0.9 released"
```

Same alterations is made for `autopep8`