Remove tree/count fields from Context and Query models

Fix #194.
These methods are cacheable:

```python
from avocado.core.cache import cached_method

@cached_method(data_version='modified')
def count(self, *args, **kwargs):
    # ...
```

This will use the `modified` datetime as the *version* for the cache key. Anytime the context changes, the cache key will change.