Fixed warning on SHOW SLAVE STATUS

Hello, 

This PR fixes warning that occurs to us running datadog for monitoring mysql slave. Here is example:

2014-01-28 11:10:16,760 | ERROR | dd.collector | checks.mysql(mysql.py:233) | Error while running SHOW SLAVE STATUS
Traceback (most recent call last):
  File "/usr/share/datadog/agent/checks.d/mysql.py", line 226, in _collect_dict
    self.gauge(metric, float(result[col_idx]), tags=tags)
TypeError: float() argument must be a string or a number

This is not a big problem - but it's annoying while running "datadog-agent info" command and also garbage collector.log.