API improvements ideas

[jglaszka]

We talked with @bartlomiejgraczyk and we proposed some ideas for tradukisto:

• cleanup of API - currently every class is public. We can encapsulate packages and expose publicly only needed API.
• research for language families - how are they similar and which code can be generic for them - for example many slavic languages have identical logic for differenting if number should have single or plural form.
• enum for chunking - many languages uses 3 digits chunking, indian has other chunking logic, probably asian languages can have own procedure too
• configurable currency for languages - for example as was done by @luismateoh in
  Add spanish support #111 (comment)
• BigDecimal converter - without currencies
• merge LongConverter and IntConverter into one class with two methods

[jglaszka]

• warn users if they try to access unimplemented method (it returns null now)
• generic unit test with predefinied test cases - contributors will need only to complete the list - without copy pasting etc - done in test refactor #141