Handle JSX files without the JSX pragma

I kept getting "Unclosed regular expression" errors from JSHint in my JSX files, so I stepped through the code of this package and found the reason. It might not be best practice, but we don't use the ```/** @jsx React.DOM */``` pragma, but simply use .jsx as the file extension for any React component.

This pull request checks if the current file's grammar is JSX and if it's missing the pragma. If so, add the pragma at the beginning of the string passed to the linter and after JSHint has finished, substract 1 from the line number of each error to account for the line that was added.

I hope you'll consider the suggestion. If there's something off with the PR, let me know and I'll try to fix it!