units: use "radian" for si/cgs units for angle?

Hi,

A suggestion: I think the si and cgs units for angle should be "radian"
For SI, see http://physics.nist.gov/cuu/Units/units.html
in particular table 3 and its footnotes a,b,c.
Hence,
```
import astropy.units as u
(1*u.degree).si
```
should yield the same as `(1*u.degree).to(u.radian)`
-- Marten
p.s.  Currently, .si and .cgs do nothing to angles, which I think is a poor choice; when one does `quantity.si.value`, the outcome should be independent of the unit of quantity.