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.
Don't worry about this right now, but long term, probably worth refactoring some of these "shared" arguments into a single method?

`add_common_arguments(parser_allocation)`

```python
def add_common_arguments(parser):
  parser.add_argument('-T'...)
```