Introduce units.cycle (2 pi radians) [note: initially considered u.pi]

This may be silly (@wkerzendorf certainly thought so...), but to me it made sense to introduce `u.pi`, so that one can do things like:
```
import astropy.units as u
p = 3.5 * u.hr
t = 1000 * u.s
phase = 2*u.pi*t/p
```
This example would be for eventual passing to functions, e.g., `np.cos(phase)`. This will be particularly useful if trigonometric functions insist on quanties with angle units (which I think they should).  The one disconcerting thing is the units of `constants.h/(2*u.pi)` would not match those of `constants.hbar`, but a similar inconsistency exists now, where people who write `2*np.pi*t/p` have to realise the `np.pi` carries a unit here, which they need to add.

Overall, my sense was that there was no harm done in adding `u.pi`, and I personally would like the convenience.