fixed style and mistakes
added gradle
added extractByKey and decreaseKey methods
refactoring and clear code
added test and refactoring
fix using of key instead dist
using assertArrayEquals and added check for null to merge method
Merge branch 'lab1-tdd-leftist-heap' of https://github.com/dmitry-abramov/agile-course-practice into lab1-tdd-leftist-heap