Support nil fields for pointers to primitives (uber-go#758)

This adds support for building fields from pointers to primitive types
we already accept.

These new field constructors accept and handle `nil` values in addition
to the non-nil primitive values.

For nil values, we fall back to the behavior of `zap.Reflect(..)`,
which we optimize for in the JSON encoder so that we don't use
`encoding/json` just to build the string `"null"`.

Resolves uber-go#753