Fix problem in MONTHNUM and add DATESTAMP_EVENTLOG pattern and 

Whilst trying to get Windows Event Log data into logstash I had some problems with the the timestamp format Windows uses.

Taking an example value (the TimeGenerated field provided by the eventlog input):

20131218141650.687853-000

And using:

%{YEAR}%{MONTHNUM}%{MONTHDAY}%{HOUR}%{MINUTE}%{SECOND}

I get a %{MONTHNUM} value of 1, but it should be 12. This is because the MONTHNUM pattern allows for single digit month values. I created a new pattern which removed this flexibility:

MONTHNUM2 (?:0[1-9]|1[0-2])

This, along with the pattern below works as I would expect:

DATESTAMP_EVENTLOG %{YEAR}%{MONTHNUM2}%{MONTHDAY}%{HOUR}%{MINUTE}%{SECOND}

I'm sure there's a better way of implementing this but I think this is a genuine problem.
Merge pull request #886 from tjrobinson/patch-1

Fix problem in MONTHNUM and add DATESTAMP_EVENTLOG pattern