Address missing `months` time unit

The new `--timestring` option allows for monthly index patterns, e.g. `logstash-2014.07`, but because python's `timedelta` function does not recognize months as a time unit, this functionality was omitted from the 1.2.0 release.  

This PR addresses this completely, with tests.  It also patches a few other nits.
No, because `date` objects and `datetime` objects are different.  I have to reference year and month separately here, and I also want each month to be calculated based on the 1st.