Adds JSHint gem

Adds JSHint gem and configuration file.
Cleans up JS according to JSHint settings

Performs general cleanup on JS files according to JSHint settings,
including adjusting indentation, equality conditions, and adding
missing semicolons.
Converts tabs to spaces

Converts each indentation tab to 2 spaces to conform to best practices.
Updates JSHint gem in use

Updates the JSHint gem to jshintrb from jshint. I have not been able to
find a satisfactory gem for jshint, but this configuration is the one
I’ve found that doesn’t throw errors originating from the gem itself.
Lints JS through JSHint

Corrects formatting issues in JavaScript as exposed by JSHint.
Move jshintrb gem to development group

Moves jshintrb gem to development group instead of the assets group in
the Gemfile.
Adds error handler for JSHint rake tasks

Wraps the JSHint tasks in error handling condition so rake doesn’t
abort when JSHint rake tasks can’t be initialized.
Moves JSHint task to its own task file

Moves JSHint task from the main Rakefile to /lib/tasks directory.
Adds more comprehensive code comment around teaspoon tasks in Rakefile.
Updates rake file

(For some reason this didn’t get into the prior commit.)
Adds commenting to rake file for the teaspoon gem.
Updates Readme commit section

Adds JSHint instructions to contributing section and performs minor
formatting on contribution section.
Merge pull request #356 from codeforamerica/jshint

Jshint