Populate Makefile with more refined targets.

* Rename `style` target to `check`.
* Split `check` to multiple subtargets.
* Factor execute flag check into separate script.
* Factor jshint check into separate script. Use standard file for
  JSHint configuration.
* Factor new line check into separate script.
* Factor pylint check into separate script.
* Add `unittest` target. Restrict it to the tests available in the
  test folder.
* Do not clutter Travis output with make commands.
Remove checkstyle script.
Remove redundant database file.