MarshalText for null.String

forgot to add this

fix godoc for IsZero

it had the old behavior from v2

JSON "" shouldn't unmarshal to a null null.String.

That's what the zero package is for.
This was a "feature", but inconsistent with the rest of the null package.

MarshalText, UnmarshalText for Time

prefer JSON errors when unmarshaling, better docs

Invalid JSON input will return a *json.SyntaxError instead of a cryptic error
from this package.

avoid loss of precision in JSON unmarshal

Merge pull request guregu#2 from guregu/errors

UnmarshalJSON should return an error for invalid types

fix grammar in godoc

remove nuller pkg

null version 1.0