Add jshint configuration.

Disable "undef" rule because unit-tests are referencing global variables
defined in test/support.js, which jshint cannot pick up.

/to: @ritch @raymondfeng please review
/cc: @Schoonology

See also [Code Style Guide](https://strongloop.atlassian.net/wiki/display/SL/Code+Style+Guide) on our wiki.