Fix wrong test for 'hasPathSum'.

Looking for path sum that didn’t exist.
Fix another wrong test for 'hasPathSum'.

Need to create a tree manually to get it as it should be, as it’s not a
binary search tree.
Add convenience method 'isLeaf'.
Complete Exercise 7.
Refactor tests. Separate exercise tests to own file.
Refactor tests. Improve assert usage.
Merge branch 'master' of https://github.com/seriema/tree.js

* 'master' of https://github.com/seriema/tree.js:
  Fix Travis-badge
  Update README.md
  Update README.md