Update server side code and introduce couple of changes

[michaloo]

@unity let me first try to use it and confirm that it all works.
Things I've changed:

1. separate babel configuration for client and server
2. for server transpilation use flow-strip plugin and transpile everything down to node 8.9 (there are two things I needed to change after dropping stage-0 - object spread ({ ...object }) - es6 only allows to spread array ([ ...array]) and named exports needs {} around the name
3. I've removed dotenv and replaced it with dotenv-cli which allows you to do $ dotenv npm startdev - which will load the envs but doesn't need to be included in production code -> I think you have the same dev easiness and separation of code responsibilities
4. added structure for unit and integration tests
5. added basic structure for connector lib
6. added flow related deps and adjusted config

now I need to test it and verify everything works smooth.