Consider moving date parsing behaviour out of options.display.xFirst
https://github.com/pydata/pandas/pull/3314#issuecomment-16300121, `options.display.dayfirst` modifies parsing behaviour in `parse_time_string`, which controls "smart slicing" for timeseries.
Consider seperating the display and parsing behaviour. the parsing
behaviour belongs in `options.mode`. with a warning in 0.12 and the change
in 0.13 if we do it. 

opinions?

```
In [2]: pd.describe_option("dayfirst")
display.date_dayfirst: 
: boolean
        When True, prints and parses dates with the day first, eg 20/01/2005
```

This has ancient roots: https://github.com/pydata/pandas/commit/6876725e#diff-7c0967e4a4ffec815d7be12079eaed6aR126
