Change OpenCV import to make it an optional dependency

skimage/detection/_template.pyx

@@ -3,9 +3,9 @@
 import cython
 cimport numpy as np
 import numpy as np
-import cv
 from scipy.signal import fftconvolve
 
+
 cdef extern from "math.h":
     double sqrt(double x)
     double fabs(double x)

skimage/detection/template.py

@@ -1,9 +1,15 @@
 """template.py - Template matching
 """
 import numpy as np
-import cv
 import _template
 
+try:
+    import cv
+    opencv_available = True
+except ImportError:
+    opencv_available = False
+
+
 
 #XXX add to opencv backend once backend system in place
 def match_template_cv(image, template, out=None,  method="norm-coeff"):
@@ -23,6 +29,8 @@ def match_template_cv(image, template, out=None,  method="norm-coeff"):
         Correlation results between 0.0 and 1.0, maximum indicating the most
         probable match.
     """
+    if not opencv_available:
+        raise ImportError("Opencv 2.0+ required")
     if out == None:
         out = np.empty((image.shape[0] - template.shape[0] + 1,
                         image.shape[1] - template.shape[1] + 1),