Loop unrolling threshold was too small

it prevented unrolling on this simple example:

    #pythran export conv(float[][], float[][])
    import numpy as np
    def conv(image, weights):
        result = np.zeros_like(image)
        for i in range(1, image.shape[0]-2):
            for j in range(1, image.shape[1]-2):
                for ii in range(-1,2,1):
                    for jj in range(-1,2,1):
                        coef = weights[ii+1, jj+1]
                        result[i,j] += image[ii+i, jj+j] * coef
        return result
Add test annotation for test tuning

* Thanks to pytest mark, we can select tests with -m module (to execute
* only module annotate class) and -m not module (to execute only not
* annotate class)
Merge pull request #295 from serge-sans-paille/increase-loop-unrolling-threshold

Loop unrolling threshold was too small
Merge pull request #286 from pbrunet/module_test

Add test annotation for test tuning
Merge pull request #289 from pbrunet/real_loop

Real loop
Add pattern replacement for len_set
Add new pattern modification
Fix distutils tests
Reword pattern matching
Fix is_assign name