Only use colorama when curses unavailable

Only use colorama when curses unavailable

This also uses dimmer colors for Windows. This can be tested with
the following script:

```python
import logging
from tornado.log import enable_pretty_logging

logger = logging.getLogger()
enable_pretty_logging()
logger.setLevel(logging.DEBUG)

logger.info("INFO")
logger.warning("WARNING")
logger.debug("DEBUG")
logger.error("ERROR")
print("normal")
```