be robust to dealloc while doing some network
Merge pull request #46 from fotonauts/dealloc-robust

Make -dealloc and cancellation more robust.

This nullifies the delegate at -[OFFlickrAPIRequest dealloc] and also
takes care of an issue in LFHTTPRequest where NSTimer may be the last
instance that holds onto its delegate (since NSTimer is one of those
special cases that retain the delegate).

Credit to Jérôme Lebel.