count() doesn't apply limit()

A query with an explicit limit doesn't apply the said limit on a count request.

In the attached unit test, the statement:
```python
TestModel.objects(TestModel.test_id == 0).limit(2).count()
```

execute the following CQL query:
```sql
SELECT COUNT(*) FROM cqlengine_test.test_model WHERE "test_id" = :0 {'0': 0L};
```

Here, I expect cqlengine to produce:
```sql
SELECT COUNT(*) FROM cqlengine_test.test_model WHERE "test_id" = :0 {'0': 0L} LIMIT 2;
```