Provide an option lazily instrument javascript files (if the file hasn't been modified, don't reinstrument)

Hi,

We've been trying out this library on [a web application](http://labs.ft.com/articles/the-ft-web-app/) with a very large code base (300+ javascript files) and it's working really well, but it take a little while to run through all the files.

To try to address the speed I've added an option to only instrument files that have been modified since the last time they have been instrumented.

I'm not sure `lazy` is the right word but I can't think of a better one at the moment...  I'd be very happy to change it if you have any suggestions.

Looking forward to hearing your feedback!

PS. My code editor trims the ends of lines (which is what happened on lines 20 and 39).  I can undo this if it's a problem.