Don't rely on non-standard 'INT32_MAX' / 'INT32_MIN'.

Define safe constants to use when padding enums.

Fixes #80.
Can you change them to use "kint32min" and "kint32max" (defined [here] (https://github.com/google/protobuf/blob/master/src/google/protobuf/stubs/common.h)) and remove the inclusion of \<stdint.h>?