Add method on NSString to get a trimmed string.

- The trimming removes any whitespace in the beginning and end of the
receiver.

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>
Add first step towards showing a filtered list of questions.

- That is, the first step towards being able to display questions
related to a tag.

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>