Add Len() assertion

Added `assert.Len(t, obj, l)` which assert `len(obj) == l`.

Of course, `assert.Equal(t, len(obj), l)` do same assertion.

However, I use assertion of `len(obj) == l` often (especially `len(obj) == 1`).
I think `assert.Len(t, obj, l)` very useful to restrict loss of gopher's life
and is not so ad hoc because `len()` is built-in.