Implement assertion error messages and more

[BridgeAR]

As a follow-up of #44 it's still required to use util.inspect. That's currently not the case and a lot of message tests fail because of that.

[BridgeAR]

This issue is meant as general tracker for things that should be implemented after #44 landed.

As such we have to decide if we want to keep on supporting IE or not. I personally would drop the support in another semver-major. Otherwise it's difficult to keep up with changes and implement new features.

[BridgeAR]

If someone would still want to use the IE they would have to transpile the code with babel accordingly and hope everything works well. I guess most features that someone in the IE would use would keep on working and it's mainly the tests that are difficult to keep up (like new language features are not required in the IE so code branches working specially for new types would not be taken etc).

[rvagg]

@BridgeAR are you maintaining this library these days? It could do with a new sync I think. throws, rejects and others seem to be outdated or missing (I've resorted to manual polyfills for both in different situations). Also yeah, error messages are pretty wonky when seen side-by-side with Node's.