BUG: fixing tseries plot cursor display, resolves #5453
Fixes #5453

I am not sure how to test a mouse-over, and the original commit 922c6102eebb7347cea587fffc4795a3ca3e73b2 introducing this functionality did not have tests either, which is sadly probably how this got broken. @willfurnass do you happen to have any ideas for how to test this functionality, maybe from when you first looked at this?

The new code doesn't display timezone information like the old code once did (before it was broken) as Periods don't seem to have timezone info, and all `tseries/plotting.py` plots are Period-based. One could perhaps do some minor acrobatics to convert the tz-naive start time of the Period into a tz-aware Timestamp, I can research that if people feel strongly about it.

Hmm, actually I kind of like leaving the display as a Period - works nicely for Weeks, Quarters, etc.
 Maybe you could try to test it by making a plot, and then feeding manually data to `ax.format_coord` and test if this is formatted well. Something like:

```
s = pd.Series([1,2,3], index=pd.date_range('2014-01-01', periods=3, freq='H'))
ax = s.plot()
string = ax.format_coord(some_data)
assertEqual(string, expected)
```

and that for different frequencies. Just an idea, don't know if this will really work.
 Yup that works - I've added a test along those lines that would have caught the original issue.
 @jorisvandenbossche @TomAugspurger ok with this...

@rosnfeld ping when green
 @jreback ping
 @TomAugspurger if this is ok
go ahead and merge
 Thanks @rosnfeld
