Release v4

[guregu]

I'd like to work towards releasing a new version.

v3 stuff

• Merge Added Equal method to nulltypes. #36 because it's backwards compatible, need to add zero support first.

v4 stuff

• Fix Why does null.Time text-marshaling return null string? #47 (backwards incompatible)
• Take the opportunity to get rid of some cruft? If we drop support for marshaling from sql.NullXXX JSON objects (as in

  null/int.go

  Lines 70 to 71 in 80515d4

  	case map[string]interface{}:
  	err = json.Unmarshal(data, &i.NullInt64)

  ) we can speed up the library a bit and avoid having to unmarshal into interface{} first. I doubt many people use this anyway.

[limoli]

Waiting for this amazing version :)

[guregu]

Done, sorry for the wait.