Generated equals(Object) method parameter should be @Nullable

In AutoValue,

``` java
  @Override
  public boolean equals(Object o) {
  // ...
```

should be

``` java
  @Override
  public boolean equals(@Nullable Object o) {
  // ...
```

I think it's better form, and without it, AutoValue doesn't play nicely with Guava's AbstractPackageSanityTests.
