Bug 885713 - propose a "make hint" that would use jshint instead of gjslint r=ochameau,rik

This adds
* a default .jshintrc configuration file
* a `make hint` launcher
* a way to launch the linters (including the old lint linter) for a specific app
  only.
* you can also use another jshint configuration file
* use .jshintignore file as main exclude file, from both gjslint and jshint
* the pre-commit hook uses a makefile goal now
* remove the node_modules directory in really_clean
* use real files instead of phony targets for node modules and git hooks