BF: strange error when passing bool as mask type

np.bool_ as very reasonable mask input type caused an error for
np.nan_to_num.  Only run nan_to_num if floating point mask type.
DOC+TST: add examples to pca docstrings

Add doctests and some commentary to ``pca_image`` and ``pca``.
Merge pull request #243 from matthew-brett/pca-mask-fix

MRG: strange error when passing bool as mask type

np.bool_ as very reasonable mask input type caused an error for
np.nan_to_num. Only run nan_to_num if floating point mask type.

Add examples in pca and pca_image docstrings.