added py::ellipsis() method for slicing of multidimensional NumPy arrays

This PR adds a new py::ellipsis() method which can be used in
conjunction with NumPy's generalized slicing support. For instance,
the following is now valid (where "a" is a NumPy array):

py::array b = a[py::make_tuple(0, py::ellipsis(), 0)];

tests/test_numpy_array.py

@@ -408,3 +408,9 @@ def test_array_create_and_resize(msg):
     a = m.create_and_resize(2)
     assert(a.size == 4)
     assert(np.all(a == 42.))
+
+
+@pytest.unsupported_on_py2
+def test_index_using_ellipsis():
+    a = m.index_using_ellipsis(np.zeros((5, 6, 7)))
+    assert a.shape == (6,)