add more isreadable/iswriteable/isreadonly functions

[stevengj]

We already export these functions, this just adds definitions for more stream types.

(It would be nice to add them for File objects too, but that seems like it requires one to modify libuv to add a new function that calls fcntl(fd, F_GETFL) or the Windows equivalent thereof.)

[stevengj]

PS. Note that we aren't entirely consistent on the spelling. We have iswriteable but IOBuffer.writable, for example. According to Google, the spelling "writable" is more common, so we might consider switching and deprecating the old spelling.

[pao]

"Writable" appears to be the preference in print as well.

[Keno]

I will merge this an we can do the rename/deprecation in a separate commit.