Add jshintrc
Move jshint rules into jshintrc and include from Gruntfile.
Add async parsing option.

In my testing, the major time sink is parsing. This commit adds a setTimeout() around parsing
of each item so control can return to the browser. This increases the total time it takes to finish
a screenshot but will not freeze the browser when it does. This is a good option when e.g. doing
error reporting, where you might not want to freeze the browser while sending debugging information
back to your server.