Add additional metrics to Apache httpd collector.

This allows the Apache httpd collector report additional metrics about
workers by parsing the "Scoreboard" from mod_status; previously, the
Scoreboard was ignored.

In addition to reporting "IdleWorkers", the collector now reports
metrics for: 'StartingWorkers', 'ReadingWorkers', 'WritingWorkers',
'KeepaliveWorkers', 'DnsWorkers', 'ClosingWorkers', 'LoggingWorkers',
'FinishingWorkers', and 'CleanupWorkers'
Merge pull request #520 from jswank/master

Add additional metrics to Apache httpd collector