call import_array() at load time

All extensions that call into NumPy should call this function, see
http://docs.scipy.org/doc/numpy/reference/c-api.array.html#importing-the-api
(also gets rid of a compiler warning).
Update readme.rst
ignore HTML files from cython -a
fix some integer types in Cython code (64-bit safety)

NumPy indexes should be of type cnp.npy_intp. Python indexes are
Py_ssize_t. Using bint for a boolean gives better typechecking.
Merge pull request #27 from larsmans/fixes

Minor fixes