python: Ensure that we pass utf-8 encoded string to libnotmuch

If we use unicode objects, libnotmuch would not cope with null bytes in
the byte array, so we need to make sure they are nicely formatted as
utf-8.

Introduce a helper function _str which does this throughout the code.

Patch slightly modified by Sebastian Spaeth.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>