Add database operations.

```python
from sqlalchemy_utils import database_exists, create_database, drop_database

# Check if the database referenced in a sqlalchemy URL exists.
assert not database_exists('postgres://postgres@localhost/db_test_sqlalchemy_util')

# Create that database.
create_database('postgres://postgres@localhost/db_test_sqlalchemy_util')
assert database_exists('postgres://postgres@localhost/db_test_sqlalchemy_util')

# Now drop it.
drop_database('postgres://postgres@localhost/db_test_sqlalchemy_util')
assert not database_exists('postgres://postgres@localhost/db_test_sqlalchemy_util')
```

The functions have full support for postgres, mysql, and sqlite. I don't know much about oracle or other db engines but it *should* work there as well.