Adds _.isFile and _.isBlob

This adds `_.isFile`, which returns true for File and Blob objects, and `_.isBlob`, which only returns true for the latter. Maybe this is too soon, since only IE 10+ seems to support File and Blob, but these have been helpful for me in a couple of projects.

Unfortunately, it doesn't seem possible to create a positive test case for File objects, as I don't believe they can be created in Javascript.