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.
Merge pull request #193 from lpsinger/error_if_nside_gt_2_29

Check that nside <= 2**29