Add a .jshintrc file and associated jshint grunt task.

This .jshintrc is adopted from the jQuery project, with some basic modifications (such as single quotes instead of double quotes). This pretty closely follows our current JS standards and rather closely resembles our PHP standards, especially the love of whitespace. The major changes are enforcing === and always using braces for if statements.

props kadamwhite, gnarf37, with mattwiebe and carldanley.
see #25187.


git-svn-id: https://develop.svn.wordpress.org/trunk@25960 602fd350-edb4-49c9-b593-d223f7449a82