Tidy up and bug fixes

[aanno]

Well, as I tried this, I stepped upon a few problems:

1. Missing documentation, hence I updated the readme.
2. A few missing @Override and @FunctionalInterface.
3. Public attributes in NGContext, hence I added getter (the setter were already there).
4. Problems with the Classloader stuff on Java ~8~ 11, hence it fixed that.
5. mvn version not set to a SNAPSHOT
6. A few missing flush().
7. ng-version command not working in development setup.
8. Some errors (like ClassNotFound) not propagated to client.

I decided to fix all that.

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[v-jizhang]

@aanno This PR should split into multiple ones. Could you do it? Thanks.

[aanno]

Ok, first round:

• improve documentation: #187
• new mvn SNAPSHOT version: #188
• refactoring: #189

After these 3 are accepted, I will work on the rest.