Performance fix for heap; correctness for _mcp

(1) normalize_indices in _mcp.pyx should convert indices to ints: fixed
(2) huge (order of magnitude) performance problems in heap.pyx from changing
lines like:
ii = (i-1)/2 # CalcPrevAbs

to:
ii = (int)(i-1)/2 # CalcPrevAbs

for some reason,  the latter goes through the python division machinery!
However, the former is invalid for cython >=0.12, so the proper solution is:
ii = (i-1)//2 # CalcPrevAbs

scikits/image/graph/_mcp.pyx

@@ -184,6 +184,7 @@ def _normalize_indices(indices, shape):
             return None
         new_index = []
         for i, s in zip(index, shape):
+            i = int(i)
             if i < 0:
                 i = s+i
             if not (0 <= i < s):

scikits/image/graph/heap.pyx

@@ -246,7 +246,7 @@ cdef class BinaryHeap:
             i0 = (1 << level) - 1 #2**level-1 = LevelStart
             n = i0 + 1 #2**level
             for i in range(i0,i0+n,2):            
-                ii = (int)(i-1)/2 # CalcPrevAbs
+                ii = (i-1)/2 # CalcPrevAbs
                 if values[i] < values[i+1]:
                     values[ii] = values[i]
                 else:
@@ -266,7 +266,7 @@ cdef class BinaryHeap:
         # track tree        
         cdef int ii, level      
         for level in range(self.levels,1,-1):        
-            ii = (int)(i-1)/2 # CalcPrevAbs
+            ii = (i-1)/2 # CalcPrevAbs
             # test
             if values[i] < values[i+1]:
                 values[ii] = values[i]