Merge pull request #509 from stefanv/contributor_guidelines

DOC: Refresh contribution guidelines.
Updated Implementation of Roberts Algorithm
Upadated Version with test added(/filter/tests/test_edge.py
ENH: Add `chelsea` to dataset.
Merge pull request #524 from stefanv/chelsea

ENH: Add `chelsea` to dataset.
updated tests for Roberts Edge Detector
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.
fix travis complaint
readability
Updated function names[Robert's Edge Detection]
Merge pull request #526 from sciunto/fixtravis
temporarily use Cython 0.18 to restore normal Travis builds
Final Roberts Edge Detector
Merge pull request #527 from JDWarner/tempfix_use_old_cython

TST: Temporarily use Cython 0.18 to restore normal Travis builds.
Clean, and implemented using horse.jpeg
Updated Test Case
Added horse.png to skimage/data and added a horse() in the _init_.py in data subpackage
End newline added and removed the .jpeg version