Fix imread from urls on some backends.

Some backends infer the image type from the extension. This fix just adds the extension to the temp filename.
ENH: Add `chelsea` to dataset.
Merge pull request #524 from stefanv/chelsea

ENH: Add `chelsea` to dataset.
BUG: Fix integer type in Felzenwalb cython.
Merge pull request #515 from tonysyu/fix/imread-url-bug

ENH: Fix imread from urls on some backends.
Merge pull request #525 from stefanv/felzenwalb_32

BUG: Fix integer type in Felzenwalb cython.