Normalizing Time format

In order to normalize how time is displayed throughout SunPy, I've defined a new configuration parameter, `time_format`, which should be used throughout SunPy whenever a time is displayed in the console or in a plot. This new parameter is now also available to the user to define as they see fit if they prefer a different format. 

I've tried to find all of the places where time is displayed but I'm sure I've missed a few.
I would suggest, however, that the default format is the ISO8601 one:
`%Y-%m-%dT%H:%M:%S`
I think it's the only one would not cause confusion to anyone, `/` makes me always wonder whether is day/month or month/day.