Added EncodeInt method for int64 values.

Summary:
This was necessary because float64 cannot hold the full precision of
a 64bit integer.

Test Plan: go test

Reviewers: andybons, jiangmingyang, petermattis

Reviewed By: petermattis

Subscribers: petermattis, team

Differential Revision: http://phabricator.cockroachdb.org/D119