Allowing for custom format of ticks that get drawn on the plot. Useful for cases
in plots where the data is a time series and the user wants to view the tick in
HH:MM format rather than time since epoch which is how the data might be stored
in a PDL object.
Correct implementation of the format_tick call so that it gets invoked at the
right time.
Using run4flat's suggestions and updating the code to have a default format_tick
and verify the user's setting if any.
Merge pull request #26 from vikasnkumar/master

Adding a custom format tick subroutine option instead of plain sprintf when the tick is drawn