Introduce sidereal_time method to Time
Also changed internal use of lon, lat: swapped order in call sequence to
match convention (and coordinates), made default None, so that error can
be raised if an local sidereal time is requested, and used Longitude/Latitude
for them so that they can be defined much more easily.
Add documentation
Use explicit order arguments in format; proper spacing in CHANGES.rst
Formal sidereal_time.__doc__ such that build_sphinx is happy
Small adjustments to sidereal time documentation
Add PR number to CHANGES.rst
Merge pull request #1418 from mhvk/time-sidereal-time

Add sidereal time.