pip+virtualenv installation fix. closes #367

For unknown reasons (probably a mix between pip, setuptools and distutils invocation), our extension source are automatically replaced from pyx to c, and then, Cython is not doing its work.
So after invocation of the Extension.__init__, force again the source field.

setup.py

@@ -140,9 +140,9 @@ if True:
             pyxl.pop(0)
         return '.'.join(pyxl)
 
-    OrigExtension = Extension
+    class CythonExtension(Extension):
 
-    def Extension(*args, **kwargs):
+        def __init__(self, *args, **kwargs):
             # Small hack to only compile for x86_64 on OSX.
             # Is there a better way to do this?
             if platform == 'darwin':
@@ -151,23 +151,26 @@ if True:
                     kwargs.get('extra_compile_args', [])
                 kwargs['extra_link_args'] = extra_args + \
                     kwargs.get('extra_link_args', [])
-        ext = OrigExtension(*args, **kwargs)
-        ext.pyrex_directives = {
+            Extension.__init__(self, *args, **kwargs)
+            self.pyrex_directives = {
                 'profile': 'USE_PROFILE' in environ,
                 'embedsignature': False}
-        return ext
+            # XXX with pip, setuptools is imported before distutils, and change
+            # our pyx to c, then, cythonize doesn't happen. So force again our
+            # sources
+            self.sources = args[1]
 
     # simple extensions
     for pyx in (x for x in pyx_files if not 'graphics' in x):
         pxd = [x for x in pxd_files if not 'graphics' in x]
         module_name = get_modulename_from_file(pyx)
-        ext_modules.append(Extension(module_name, [pyx] + pxd))
+        ext_modules.append(CythonExtension(module_name, [pyx] + pxd))
 
     # opengl aware modules
     for pyx in (x for x in pyx_files if 'graphics' in x):
         pxd = [x for x in pxd_files if 'graphics' in x]
         module_name = get_modulename_from_file(pyx)
-        ext_modules.append(Extension(
+        ext_modules.append(CythonExtension(
             module_name, [pyx] + pxd,
             libraries=libraries,
             include_dirs=include_dirs,