Added Equal method to nulltypes.

[MarkKremer]

I added the methods as discussed in #35 . The methods are called Equal instead of Equals (with -s) to be more consistent with the go library.

Interesting bits:

• Time can be compared using Equal (uses time.Time.Equals) and ExactEqual (uses the ==-operator)
• Equal is implemented for Float. The doc above the method mentions that comparing floats is not always recommended.

[guregu]

Thanks for the PR! LGTM. I’ll add the equivalent methods to the zero package and merge this soon (ETA: this weekend)

[abramche]

Any news @guregu?