fmt in TimeFormatter is now changeable
`datetime.time` objects are useful as index values, e.g. in a data frame
containing data of an experiment lasting a few minutes or hours. When
plotting such data frame, the x-axis ticks are formatted as `'%H:%M:%S'`
as standard. In most cases, either `'%H:%M'` or `'%M:%S'` contains enough
information, and the `'%H:%M:%S'` format is superfluous.

The xticks were formatted based on the fmt variable in the `__call__` method. In this commit, the tick format is saved in the `TimeFormatter.fmt`
variable. This is set in the `__init__` method to `'%H:%M:%S'`, but can
later be changed, e.g.:

```
ax = df.plot()
ax.xaxis.get_major_formatter().fmt = '%M:%S'
```
