Add debug-level logging to ajax check

The js now returns dict with relevant keys and `True/False` values (which are then logged, if True) instead of just a single bool.

Update: Should be thread-safe and less spammy; logs the message only if it's different from the last one.

Example log:
```
2014-07-17 13:11:05,885 [D] Finished filling in form (utils/log.py:411)
2014-07-17 13:11:06,426 [D] Ajax running: jquery, spinner (utils/log.py:411)
2014-07-17 13:11:13,705 [D] Ajax done (utils/log.py:411)
2014-07-17 13:11:13,753 [I] Navigating to infrastructure_providers (utils/log.py:411)
2014-07-17 13:11:21,598 [D] force_navigate to infrastructure_providers, try 1 (utils/log.py:411)
2014-07-17 13:11:21,658 [I] Navigating to infrastructure_providers (utils/log.py:411)
2014-07-17 13:11:23,807 [D] Ajax running: document (utils/log.py:411)
2014-07-17 13:11:23,936 [D] Ajax done (utils/log.py:411)
2014-07-17 13:11:25,010 [D] Provider "XYZ" exists, skipping (utils/log.py:411)
2014-07-17 13:11:25,011 [D] force_navigate to infrastructure_hosts, try 1 (utils/log.py:411)
2014-07-17 13:11:25,053 [I] Navigating to infrastructure_hosts (utils/log.py:411)
2014-07-17 13:11:30,310 [D] force_navigate to infrastructure_hosts, try 1 (utils/log.py:411)
2014-07-17 13:11:30,355 [I] Navigating to infrastructure_hosts (utils/log.py:411)
2014-07-17 13:11:33,073 [D] Ajax running: document (utils/log.py:411)
2014-07-17 13:11:33,183 [D] Ajax done (utils/log.py:411)

```
Here's what I'm thinking:
```python
in_flight = execute_script(js.whats_in_flight)
anything_in_flight = any(v for v in in_flight.itervalues())
prev_log_msg = getattr(_thread_local, 'ajax_log_msg', '')

if anything_in_flight:
    log_msg = ', '.join([k for k, v in in_flight.iteritems() if v])
    # Log the message only if it's different from the last one
    if prev_log_msg != log_msg:
        _thread_local.ajax_log_msg = log_msg
        logger.debug('Ajax running: {}'.format(log_msg))
elif prev_log_msg:
    _thread_local.ajax_log_msg = ''
    logger.debug('Ajax done')

return not anything_in_flight
```