Unit tests, refactoring, make package extensible

[keskiju]

• Create a separate atom-java-project package which parses/indexes all java related project content (build properties, classes, java sources, etc) and provides centralized java configuration. It will be used by autocomplete-java, linter-javac and other java related packages.
• Move additional logic to separate packages. Packages use services of atom-java-project and autocomplete-java.
• Replace promises with async-await
• Rewrite quick and ugly hacks
• Write unit test while refactoring code.