fix bad regression with Status type

[yannmh]

Switching Status type from String to int introduced a bad regression in Datadog agent.

2015-04-01 15:08:57,849 | ERROR | dd.collector | checks.check_status(check_status.py:776) | Couldn't load latest jmx status
> Traceback (most recent call last):
>  File "/home/vagrant/workspace/dd-agent/checks/check_status.py", line 759, in get_jmx_status
>    check_data[check_name]['statuses'].append(get_jmx_instance_status(instance_name, status, message, metric_count))
>  File "/home/vagrant/workspace/dd-agent/checks/check_status.py", line 688, in get_jmx_instance_status
>    return instance_status
> UnboundLocalError: local variable 'instance_status' referenced before assignment

This PR reverts the change introduced in:
https://github.com/DataDog/jmxfetch/pull/38/files#diff-c6d094e1c3c4dcd3b460b535995a43ccR14

@wang-arthur @remh

[remh]

lgtm!