skip unsafe tests for bigendian architectures. (gogo#272)

* skip marshal and unmarshal tests on big endian architectures if message has generated unsafe methods

* equal and compare tests now also check for bigendian architectures when the message uses the unsafe marshaler or unmarshaler

* skip packed unsafe issue21 if cpu is not little endian

Updateded Readme: formatting

fix for issue 158

Merge pull request gogo#97 from tamird/protoc-gen-combo-errors

Make `protoc-gen-combo` actually report errors