Check that nside <= 2**29

`Healpy_Base2` works up to `nside = 2**29`. Raise a `ValueError` if `nside` is out of bounds, instead of silently producing incorrect output.

The `TestPixelFunc.test_ang2pix_nest_outofrange_doesntcrash` unit test was failing on Mac OS X 10.9 anyway: the returned `theta` contained non-finite values, but `phi` did not.