Methods and documentation for all-sky projections

[telegraphic]

Many users want to do driftscan simulations using a beam model and a sky model. This requires gridding the beam into healpix, or projecting the healpix image into orthographic coordinates (the former being preferable).

Also, the documentation on healpy's rotation is rather sparse (e.g. the signficant confusion in #4). It seems many astronomers find themselves hitting their heads against the same wall -- we should try and help!

Adding in some specific methods for beam x sky simulations would also be useful.