Added documentation for each keyword.

Also used __all__ to limit what methods RF recognizes as keywords.
Other to do is to try and learn how to make a built-in
Cell Should Contain assertion.

Signed-off-by: Ika Belerma <ika.belerma@gmail.com>
Added ability to retrieve the sheet names.

That was too easy.

Signed-off-by: Ika Belerma <ika.belerma@gmail.com>