support assert.Nil or assert.NotNil

[ningmingxiao]

support assert.Nil or assert.NotNil

[cbandy]

Nil and NotNil are spelled assert.Assert(t, v1 == nil) and assert.Assert(t, v2 != nil) and produce clear messages:

assertion failed: v1 is not nil
assertion failed: v2 is nil

If you want to see the not-nil value, use assert.Equal(t, v1, nil):

assertion failed: somestr (v1 string) != <nil> (nil <nil>)

As discussed in #49, there aren't any other values to show when v2 != nil fails, v2 is nil says it all.