Introduce units.cycle as part of astropys.py
(rather than si.py, since cycle is a non-SI unit recognised for use in
astronomy: http://www.iau.org/science/publications/proceedings_rules/units/)
Add test that u.cycle.to(u.rad) is close to 6.28...
Merge pull request #1160 from mhvk/units-pi

Introduce units.cycle (2 pi radians)