bump version to v6

Merge pull request #77 from guregu/v5-dev

v5: More types, generics

update readme for v4

zero: add Equal and ValueOrZero methods

add IsZero to Time

Merge branch 'or-zero'

float: return error when marshaling NaN or Inf

move test badge to CircleCI

MarshalText for null.String

forgot to add this

fix godoc for IsZero

it had the old behavior from v2