The plugin should not get stuck on files that take JSHint a long time to process

The problem is for large files and sometimes other files, like minified files, the JSHint algorithm will run for a very long time. In the profiler it appears to be taking time inside Rhino.

For minified files, it's probably a problem with JSHint internally. For very large files taking a long time is probably not avoidable.

Here are the desires in decreasing priority
- When shutting down Eclipse, JSHint must be interruptable so that Eclipse can shut down cleanly
- JSHint-Eclipse should not be run on files for which it will not finish quickly (by default)

Possible approaches
- Don't parse files over a certain size (doesn't handle the case for small files that take a long time)
- Abort parsing a file forcibly after it has been running for 2 minutes (or if the ProgressMonitor has been cancelled) - implementation possibly related to issue #36.