Add `plot_sky_24hr`

[eteq]

As discussed in our telecon today, we merged #30 without any special behavior for scalar inputs (they are just treated like single-element vectors). Instead, there should be a new function called something like plot_sky_24hr or plot_sky_night or plot_sky_somethingelse which plots a target +/- 12 hr from a provided time. This should actually use plot_sky internally, probably just accepting a target, time (which now must be a scalar - should raise an exception otehrwise), and any further keyword arguments would just be passed to plot_sky using **kwargs.

Additional enhancements discussed in the telecon:

• Include a delta keyword (which defaults to 1 hour), controlling the spacing between the points.
• Add a keyword for annotations showing the direction of rise/set like Added plot_sky function and example notebook. #30 (comment)
• Add a keyword to add an extra marker for the "middle" time (i.e., the time that actually was passed in).

[cdeil]

@jberlanga , @eteq - Status?

Is this feature request addressed by the current plot_sky function?
If not, does someone have time to address it today or should we move the issue to 0.2?

[jberlanga]

@cdeil -- I think we should move this to 0.2.

[cdeil]

@jberlanga – You should have permissions now to label and move milestones yourself ... go ahead!

[bmorris3]

This was closed by #124.