Split Elm code into Main and helper modules

[tad-lispy]

As discussed here https://github.com/niteoweb/salarycalc/pull/29#issuecomment-485755681 it would be good for tests. The modules should probably be focused around types, maybe:

• Cities
• Careers (including Role type)

Then all logic could go to SalaryCalculator (functions like salary, commitmentBonus). The Main module would contain main, init, update, view and their helpers.